OK, I don't have time to answer this as I want to. My background is in psychology, and I fell into role playing games when I turned 10 in 1976. So by the time I was in college, understanding where the term Roleplaying game really came from, I understood the critical nature of immersion, how it is the most important ingredient for game success.
And to be clear, the definition of immersion is to "Immerse oneself into the identity and Role of the part one is playing. To respond, as much as possible, as the person one is playing, not as oneself."
And before getting into the smaller details, I will dive right into the fact that the very system/game one chooses has a huge amount to do with the amount of Immersion.
Metagaming is the opposite of immersion. You use both terms, but I need to make that absolute definition from the beginning. This also means rules that encourage metagaming decrease the immersion in a game and therefore, decrease the main ingredient of a roleplaying game. The mechanics are called "Dissociated Mechanics", a term coined by Justin Alexander. This is very worth reading, because it gets into many of the larger picture issues with players being able to use in-game logic to see the world around them, as opposed to the rules forcing dissociation from in-game logic.
Once the players assume that rules are going to determine the content of an encounter or treasure (based on EL, or whatever) instead of what the environment or history of the area dictate, verisimilitude is lost.
Vreeg's Rules of Setting design are also heavily immersion related. My current campaign is 26 or so years old (started in '83). Building verisimilitude is a huge part of this.
Vreeg's first Rule of Setting Design
Make sure the ruleset you are using
matches the setting and game you want
to play, because the setting and game
WILL eventually match the system.
Corollary to Vreeg's First Rule
The proportion of rules given to a
certain dimension of an RPG partially
dictate what kind of game the rules
will create. If 80% of the rulebook
is written about thieves and the
underworld, the game that is meant
for is thieving. If 80% of the
mechanics are based on combat, the
game will revolve around combat.
- Multiply this by 10 if the reward
system is based in the same area as
the preponderance of rules.
2nd Corollary
Character growth is
the greatest reinforcer. The
synthesis of pride in achievement
with improvement in the character
provides over 50% of the
reinforcement in playing the game.
Rules that involve these factors are
the most powerful in the game.
Vreeg’s Second Rule of Setting Design
Consistency is the
Handmaiden of Immersion and
Verisimilitude. Keep good notes, and
spend a little time after every
creation to ‘connect the dots’. If
you create a foodstuff or drink, make
sure you note whether the bars or inns
the players frequent stock it. Is it made
locally, or is it imported? If so,
where from? If locally made, is it
exported?
Vreeg's Third Rule of Setting Design
The World In Motion is critical
for Immersion, so create 'event
chains' that happen at all levels of
design. The players need to feel like
things will happen with or without
them; they need to feel like they can
affect the outcome, but event-chains
need velocity, not just speed.
Vreeg's Fourth Rule of Setting Design
Create motivated events and
NPCs, this will invariably create
motivated PCs. Things are not just
happening, they happen because they
matter to people (NPCs). There is no
need to overact, just make sure that the
settings and event-chains are
motivated and that the PCs feel
this.
Vreeg's Fifth Rule of Setting Design
The Illusion of Preparedness is critical
for immersion; allowing the players to see
where things are improvised or changed
reminds them to think outside the setting,
removing them forcibly from immersion.
Whenever the players can see the hand of the GM - even when the GM needs to change things in their favor -
it removes them from the immersed position.
(Cole, of the RPGsite, gets credit for the term).
Remember that part of immersion is the lack of feeling walls around and rails under the characters. This means that the players should not feel that there are things that their character cannot do solely because of the rules or the GM's mindset. The job of the GM is to enable roleplay, not to inhibit it.
This also means the GM must be as immersed as the players, or more.
Another big-picture thing that may irk some folk who sell stuff is that published settings can hurt immersion. They don't destroy it; but when the players have a lot of knowledge about a setting that their character would not have, this increases the opportunity to use it, consciously or unconsciously. Similarly, if your setting has its own bestiary that the characters learn as they go along, or at least a lot of homebrew tweaks, the players get used to working with the in-house data and not trusting the published sources.
If you have done all of this larger-scope stuff, the smaller scope stuff becomes easier. As a GM with miles on the tires, I find that playing up the level of knowledge my NPCs might have and do not have helps keep the players in the same mindset. Players key heavily off the way the GM plays their NPCs. They won't do the funny voices or the mannerisms if the GM does not, and if the GM is particularly careful about what their NPCs know and don't know, especially verbally, the players emulate this.
The halberd and glaive are there because D&D has a history of offering a wide variety of codified polearms. Namely 2e and previous, which 5e strives to emulate in many regards.
It's fairly likely that someone on the design team, or if not them, someone that someone on the design team talked to, thinks that D&D is not D&D without some variety of polearms to choose from. This could be just "one of those things" (commonly called "sacred cows" in jargon) that becomes a tradition of the franchise and outlives its usefulness by virtue of the fact that many players are familiar with it.
To demonstrate my point, it's easiest to again refer to the 2e Arms and Equipment book, as @nitsua60 did, but also in addition to the core rules. Here's a list of the different polearms codified in the core rules and A&E:
- Glaive
- Halberd
- Lucerne hammer -- like a halberd, but with a hammer.
- Guisarme -- a peasant's weapon specialized in dismounting knights. It's defined by a hook on one side, usually with a spear tip emerging from the hook as well.
- Longspear
- Ranseur -- a spear with a crossguard, like a trident but with unsharpened, shorter side-points.
- Scythe
- Trident
This is pretty exhaustive even considering D&D, and even considering that it's spread over two books. Remember, we're dealing with a subset of two-handed melee weapons. 3rd edition, as most are well aware, did not make any meaningful attempt to curb the amount of codified rules. Pathfinder continues the tradition; a quick glance at the Weapons table in the d20pfsrd confirms that, even going so far as to make the arguably-pointless-in-real-life distinction between the bec de corbin and the lucerne hammer.
In fact, D&D's codification of long weapons goes all the way back to OD&D 1e at latest. Gary Gygax included Appendix T to AD&D 1e Unearthed Arcana (1985) which was an extensive discussion of pole arms that included citation to four different text books about medieval weapons. He ended up including thirteen (that is 13) different varieties of polearms in the original D&D 1st edition Player's Handbook. Even before that, Gygax had provided polearm supplement rules for Chainmail, D&D's wargame predecessor, via the wargame magazine Strategic Review, second issue. Many thanks go to Korvin Starmast for providing the information.
Things like this seem simple and pointless when viewed from an outsider's perspective, but it's just one of those things that gives D&D its character, something that sets it apart from other games, even if it is only a small thing. Many people are sad to see those defining characteristics go.
Best Answer
The most polite response will not be a 'canned' response, as it will address your reasoning for saying "no". Even if the reasoning is "I am trying to limit the number of house rules" players don't like when they have an idea they think is good, but it gets shot down without justification (from their point of view).
That being said...
I get the impression this response is meant for a text message or email?
As with emailing in the workplace, messages containing no punctuation, or only 'periods', are often misconstrued as having a harsher, less empathetic tone. Throw in an "I'm sorry" or an exclamation point to soften the tone of your message.
A 'canned' response that comes off much more polite would be: