Humans get both the +2 and +1 and the polarm feat.
Having a proper 10 d weapon especially with reach shores up the melee range power these classes have. There are quite a few classes and architypes that have strong short range spell but dont have good weapons to fall back on.
Humans dont get any feat. :-/
What they get is Proficiency with: Spears, Pikes, Halberds and Glaives.
What Proficiency do is it adds +2 (at level 1, more later) to your hit with the weapon.
And unlocks special attacks that weapon provides.
Problem is that all theese four weapons are Str-based ... meaning your hit chance is calculated as:
Str modifier + Proficiency bonus + your d20 roll
Classes you listed were:
Nonmartial clerics particularly light clerics
Spore druids
Cheeky strength rogue builds
Abjuration transmutation and necromancy wizards
Warlocks
Draconic sorcs
Bards
I agree with Clerics ...
Particulary Light Cleric, especialy Str based Light cleric ... could profit from this proficiency.
Especialy with Spear, since that one is Vertsatile and can be used with Shield.
Druids ... no.
Druids rarely have high Str, unless you have some very specific build in mind.
Just as with Clerics, Druids often use shields, since bonus AC is great and if they have Quarterstaff, or Club ... and Shillelagh ... their hit chance is counted from Wisdom and damage dice is turned to 1d8 + Wis modifier ... and since Druid DO want to have high Wis modifier, this is easily better choice.
This spell tho dont work with Spear, and none of those other weapons is as far as i know Versatile.
Rogue ... no. O_o
I dont claim i know all Rogue builds in the world ... but since Rogue dont get multiattack, bcs they deliver their damage trough Sneak Attack ... and in order to deliver Sneak Attack damage, they have to attack with Finesse weapon ...
I find any use of Two Handed, Strength based weapon as big misstake for Rogue. O_o
Wizards in general, no matter what specialization, have rarely better Str than Int ...
And each and every Wizard should imho have prepared either some spell for close range, or at least Shocking Grasp cantrip (or i believe there are some others, this one is just my favourite) ...
Wich for one, calculate hit from Int ... wich automaticly gives you better chance ... and also its damage is growing as you level up, wich weapons dont do, bcs Wizard dont get extra attack.
On level 5 ... presuming our Wizard have 18 Int and 8 Str ...
Shocking Grasp chance to hit is +4 ... and damage is 2d8
Halbeard hit chance is +1 (-2 from Str, +3 Proficiency) ... and damage is 1d10
So not impossible to use ... but not even close to effective in any manner.
Same apply on Sorcers, except is Charisma not Intellect ...
Same apply on Warlocks, except is also Charisma not Intellect ...
There is one exception tho!
And that is Pact of Blade ... bcs as far as i know, Pact of Blade can make any weapon they hold to be their Pact Weapon ... and Larian is boosting the shit out of this Pact, so this combination, while still odd in PnP (but teoreticaly usable) may be even quite interesting in BG-3!
And finaly Bards ...
Same story as Sorcerer.