Greenbound Summoning works exactly as it says it does: you get a normal creature from the list, augmented with the Greenbound template. It “has” the LA/CR adjustment but that doesn’t really mean much of anything to a summoned creature anyway.
On the one hand, the Greenbound template is over-LA’d. Like most LA-carrying options, Wizards apparently decided it didn’t like the idea of players choosing them (but also apparently didn’t want to just flatly tell them “no”), so it chose LAs that are extremely high: they are there to discourage the option, and punish those players who do take it. Compare the LA adjustment to the CR adjustment, for example.
On the other hand, while Greenbound may not really be worth LA +8, it certainly is worth quite a bit. Way more than the costs associated with Greenbound Summoning. As a result, the feat is absolutely one of the most powerful feats in the game, probably second only to Natural Spell for a Druid. Even if one applies the +2 spell level adjustment that @mxyzplk found, it can be ridiculously good.
Now, does that mean it is broken? That’s harder to say. It’s dramatically more powerful than other options, but that doesn’t necessarily “break the game.” That’s a bit of a semantical argument, so I’m not going to say it is or isn’t broken by choosing one of those definitions. Instead, I want to discuss exactly what about it is powerful, and how that compares to the environment it’s set in.
In this case, the Druid itself is already extremely powerful – it is one of the “Big Five” most powerful classes in the game. The Druid has excellent crowd-and-battlefield-control, and he’s a master of polymorph effects (including Wild Shape), which are fantastically powerful. Summoning is but a small part of the overall package.
At low levels, Greenbound Summoning dramatically augments this. As you note, that’s a lot of entangles and walls of thorns. Even with the +2 spell level, you still get wall of thorns earlier than you usually could. At higher levels, it’s still quite good but not game-changing: the Druid was already doing those things anyway.
So it becomes a bit of a personal thing where one draws the line of “broken.” But to me, the Druid class itself is a much greater offender than the Greenbound Summoning feat is.
The two terms are not synonymous, but from a rules perspective, they are very near it. At least a few rules citations - some from your examples, and a few more I've cited below - indicates that "casting spells without preparation" qualifies as "casting spells spontaneously", but casting spontaneously isn't the same as casting spells without preparing them.
First, to clear up the part I feel is most importent: Sorcerer and bard spellcasting is "spontaneous" for the purposes of feats like in your examples. Please see the Quicken Spell feat description on page 98 of the Player's Handbook:
Special: This feat can’t be applied to any spell cast spontaneously (including sorcerer spells, bard spells, and cleric or druid spells cast spontaneously)
While the only mention of "spontaneous" in the actual Sorcerer's entry is in a lore section regarding how they start learning magic, it's pretty clear that the rules were written where "casting a spell you haven't prepared" qualifies as "casting a spell spontaneously". Other mentions include:
- The introduction to magic, page 169
- The introduction to the bard, page 26.
This section contains speculation: It's weak, but it's there. I find it very likely that when 3.0 was first being developed, they didn't have a name for it yet (and wrote up the sorcerer's class entry before coming up with a name for their casting style) - and then that wording stuck, especially to the feats and definitions when they were ported to 3.5 with little context. I find it especially telling that while Quicken Spell definitely refers to sorcerer casting as spontaneous, the glossary specifies that spontaneous casting is a cleric and druid ability. It's a recurring problem in OGL d20 games, as there is a LOT of copy-pasta to swallow - the language evolved, but the text did not, even when "updated".
I will try to get some more detail when I can get my hands on a 3.0 corebook, but as far as 3.5 is concerned: The wording has been spotty, but at least one rules reference starting with the core PHB has always suggested that a sorcerer or bard's spellcasting has always been considered spontaneous.
However, it should be noted that the terms are not transparent in the other direction. Clerics and druids do not cast spells without preparation; they must have prepared spells in order to have spells to sacrifice to power their spontaneous casting of three letter abbreviations. While this primarily only affects corner cases, this does mean the terms are not synonymous. For example, a wizard who has taken the Elemental Adept feat (and thus is capable of casting spontaneously) still doesn't qualify for the Air Bloodline feat, which requires the ability to cast without preparation.
So to conclude, while the terms can be used interchangeably to refer to sorcerer and bard spellcasting, there is a divide when referring to divine casters (as well as casters like wizards who've taken the Elemental Adept feat) that can matter in some cases. It is probably best to think of "casting without preparation" as a subset of "casting spontaneously": if you can cast a spell using a slot you didn't prepare that spell in, whether or not you prepared any spell in that slot, then you can cast spontaneously; but you have to be able to cast a spell using a slot you didn't prepare any spell in to be able to cast without preparation. So no, the terms are not synonymous, although sorcerer spellcasting is spontaneous. Casting without preparation is always casting spontaneously, but there are many cases in which casting spontaneously is not casting without preparation.
Best Answer
Not all clerics can spontaneously cast cure spells—evil clerics and neutral clerics who choose to channel negative energy instead spontaneously cast inflict spells—but otherwise, you’re completely correct. The spontaneous spell class features of clerics and druids are there to save you the trouble of preparing those spells. You can prepare something else, and then if it turns out you need cure or summon nature’s ally more than that thing, you can still have it.
Do note, however, that every single cure or inflict spell is also much, much weaker than a typical spell of its level, and summon nature’s ally, while decently strong, has the large complication of having a “1 round” casting time (i.e. the summon does not appear until your next turn, and that only if nothing disrupts your concentration on the spell). This is likely an intentional trade-off for clerics and druids—you can get this spell exactly when you need it, but it’s not going to be “as good” in some general sense (as opposed to the specific situation you’re in) as whatever you are replacing.
There are two cases where you still could theoretically want to prepare spells you can cast spontaneously: metamagic and the healing domain. If you know you will wish to use metamagic on a spell, it is better to prepare that spell with the metamagic than it is to spontaneously cast it with the metamagic, as the latter extends the casting time of the spell.1 And domain spell slots cannot be spontaneously converted to cure or inflict, so for example if you channel positive energy and have the healing domain, for most domain slots, you have a choice between a cure spell, or another spell that will not be eligible to convert to cure later. Ultimately, though, these are both kind of moot points—the cure spells are so bad that you shouldn’t waste spell slots trying to metamagic them, nor should you take the Healing Domain.
(It is because of this weakness of the cure spells that it’s also good to buy a wand of cure light wounds to heal in between combats—it’s the most efficient healing in the game,2 it preserves your best spell slots for better things than cure spells, and between combats you’ll usually have the time you need to zap the wand several times. Many clerics only use their spontaneous cure ability in dire emergencies. It’s only once you get the fantastic heal spell that healing in-combat becomes something worth doing even when not absolutely forced to.)
In the case of cure or inflict, this makes the spellcasting take your entire turn; in the case of summon nature’s ally, it makes the summoning take two entire rounds so by the time you get the thing, the fight is probably pretty much over. Note that there is a difference between a “full-round action” casting time, as with the spontaneously-metamagic’d cure, and a “1 round” casting time, as is the case for summon nature’s ally—the former takes your entire turn but still happens on your turn, while the latter takes your entire turn as well all the time until the beginning of your next one, making you vulnerable to disruption and delaying the actual effect of the spell.
That is, in terms of hp healed per gp spent, and actually that’s not quite true as a caster level 1st cure light wounds will heal 2-9 hp (average 4.5), while a caster level 1st infernal healing will heal 10 hp (over the course of 1 minute). But infernal healing causes alignment problems for a lot of parties/clerics, and celestial healing is complete garbage thanks to its duration of 1 round/2 levels.