For your players' specific choices:
P1 has the right idea by picking a SAD class in the Druid, but has his dump stats off kilter. While dumping Strength, even down to 6, can be managed (especially for a Druid who should be able to travel light, as Druid defensive buffs are excellent and they are well-equipped to not need tons of rations, etal), he shouldn't put his racial +2 into it.
Given his array, I would have him dump Strength and Charisma -- Dexterity is needed for initiative, and Intelligence for skill points, so you can't dump either it turns out. Wild Shape is also more beneficial for P1 than he thinks -- Natural Spell comes at the same level as Wild Shape itself in PF, so he gets casting-while-shaped essentially "for free".
Along with that, he should put that +2 racial bonus into Wisdom and bask in the glow of a +4 starting Wisdom bonus.
P5 is also on the right track with focusing on archery -- the casting of the Ranger is not strong, and with two full Vancian casters in your party (namely, the druid and the wizard), his casting won't be needed for much, while the Dexterity-based skills will be quite useful in a rogue-less party. Furthermore, his choice of Charisma for a dump stat is a reasonable one -- it's not nearly as severe a dump as in P1's case, and many Rangers don't rely upon Handle Animal the way Druids do.
Regarding concerns about healing
Having played a Druid in a party where that was the sole access the party had to healing magic, by the way, I would strongly recommend your party obtain Wands of lesser vigor if you are willing to waive the [evil] descriptor on the spell or Boots of Earth (if Fast Healing 1 is enough) if they aren't, and give one to the druid and one to another character -- Druids can't convert to healing spells as they get Summon Nature's Ally instead, and spell slots are precious especially at lower levels.
In general:
You are right to be somewhat concerned about this...
The more relevant sum in this case is of attribute modifiers -- if it's less than 0, D&D 3.5e (and presumably PF (Pathfinder) as well as it's derived from D&D 3.5e -- if it's not a rule there, you can always port the 3.5e rule to your game as a houserule) has a rule that allows the player to reroll for their stat array.
In your case, it comes out as follows:
- P1: +3, +2, +2, +0, -2, -3 = +2
- P2: +3, +2, +2, +2, +1, -1 = +9
- P3: +3, +2, +2, +1, +1, +0 = +9
- P4: +3, +2, +2, +1, +0, -1 = +7
- P5: +3, +0, +0, +0, +0, -1 = +2
Which means that all of these arrays are acceptable, at least by the sum-of-modifiers rule; however, the +2 arrays are indeed non-trivial to work with.
However, character class and construction has much more to do with this than ability scores alone.
Character classes and builds in D&D generally depend on one or more attributes to do their job:
- For a Fighter, this is either Strength or Dexterity, but Constitution also helps
- Rogues are all about Dexterity, and can make use of Intelligence and/or Charisma
- Clerics and Druids rely highly on Wisdom as it is their casting stat, but also benefit from Strength and to a lesser degree Constitution.
- Wizards are all about Intelligence, but are helped a bit by Dexterity as well
- Sorcerers rely on Charisma and also receive a little help from Dexterity
However, not all classes are this simple, or Single Attribute Dependent (SAD). Some classes, unfortunately, are Multiple Attribute Dependent (MAD), which means they have features and functions that key off of different attributes:
- Rangers rely on Wisdom for their casting, but need Dexterity heavily for combat and benefit from Constitution and/or Strength.
- Paladins are equally troublesome, as they need Strength (or possibly Dexterity) for their melee combat while using Charisma for their casting and paladin abilities. (PF actually fixed the worst of this -- the 3.0/3.5 Paladin used Wisdom for casting, which was terribad as it meant the character had to have 3 good attribute scores at a minimum.)
Furthermore, some character classes can afford to "dump", or take a penalty in, certain stats:
- Fighters can generally dump Charisma, as they aren't expected to negotiate their way out of situations
- Wizards often dump Strength, as they aren't getting into frontline combat
- Clerics and druids will frequently dump Dexterity, especially if they aren't using ranged weapons or have Zen Archery -- that feat also allows for SAD archer Rangers, relying entirely on Wisdom.
Overall, this means that P1 and especially P5 must be more careful with how they build their characters. P5 is going to have to go with a fully SAD class with an easy dump stat, for instance -- if they're dead-set on a Paladin or two-weapon fighting (aka dual wielding) Ranger, I'd let them reroll because it's not compatible with their stats array. However, that array would make a reasonable Fighter, Cleric, or Druid.
P1 is a bit more interesting, because they have a double dump on their hands -- most classes only have one designated "dump stat". It is still possible to be effective with two stats dumped, even as severely as that array dumps them, but it takes care to avoid backing yourself into an unexpected corner by dumping the wrong stat. (Dumping Intelligence, Constitution, or even Charisma can have unexpected side effects in certain games -- it may be the case that characters with a low Intelligence will not be able to speak properly, low-hit-point/squishy characters will have trouble surviving first level, or a particularly uncharismatic character will be run out of town before the adventure can get off the ground.)
Footnote: these lists are incomplete -- I don't have experience with all the 3.x or Pathfinder core classes
Bonus: Racial bonuses shouldn't be used to "fill holes"
As to P1's racial attribute bonus? It should go to the dependent attribute for P1's build, not to fill a "hole" in their attribute scores -- you're basically always better off putting it in the strong suit, especially with a +2 bonus because that translates into an unconditional +1 to the derived modifier.
Best Answer
Here's My System
When looking at dice rolled values, there must be an understanding that probabilities for the lower and upper bounds are extreme (1 in 1296 and 6 in 1296 respectively). As a result, achieving the extremes of a value should be far more difficult than for those bounds in the point buy system currently in the game.
Here is a graph of the distribution for 4d6 drop lowest (Mean = 12.24, Standard Deviation = 2.85)
Take particular notice of the extreme rarity of 3, 4, and 5 values. The best way to accommodate this is to give values below the mean their own point system. Essentially there will be two parallel point systems. The basic weight of the point costs will roughly compare to the number of discrete rolls away from the medians (of 4,4,4,2... among others).
The modified point buy
Each of the six ability scores starts at 10. You get 19 points (Strength Points) to buy score values to be greater than 10, and you can get up to 10 more points (Weakness Points) from lowering certain abilities based on how much you lower them below the base score of 10.
Weakness Points
Here is a table that shows the point values for each lesser ability score.
This demonstrates the extremely low probability that you will have multiple values less than 5 or 6. Each point of weakness you expend provides one point to your strength points.
Strength Points
Strength points reverse the effect of any attribute less than 10, and here is the table for increasing past 10:
As you can see, even if you expend all 10 weakness points, it is impossible to get two 18's in this system (or even an 18 and a 17). This is because the likelihood that you roll two 18's is 0.38%. To look at the system in action, here are some sample distributions (notice that moving along the curve in the negative direction has a bigger impact since there are less discrete rolls for the values in those regions):
Note: This system may appear to take advantage of the fallacious statistical model commonly known as the gambler's fallacy. I am aware that rolling a low value (like four 1's) does not increase your chance of rolling a higher value; this system is designed to allow for adjustments above the standard system by accounting for probabilistic measures for the rolled abilities as a whole.