The IE games like IWD or BG are (fantastic) party-based games so it does not really matter because you are playing with the entire party but with some choices, you are basically dragging your party down if you want to roleplay some subpar builds. I do not expect to be uberpowerful if I want to play a beastmaster or a rogue, but I least expect the class to be playable.
(...)
If the difference in balance between classes is too steep, you find entire servers with only weapon masters and not even one bard in sight like in NWN at high levels.
As i've said many times, the solutions is to make this classes more interesting to play, not every class equally boring.
If And i an glad that you recognize that on high level NWN2 servers, nobody plays as caster due the Sawyerism on arcane classes. NWN2 is clearly Martial > Divine > arcane. With spell fixes and warlock reworked mod Martial still has his ludicrous op gear but at least casters has ludicrous op spells.
But of course, the overbearing superiority of clerics and warriors in NWN is as problematic as the superiority of arcane casters in other games.
As I said before, I understand that there always be some classes that have advantage in a campaign, because they have a lot of utility, a lot of power, or they are more suited for this campaign (In NWN2 OC you spend the last chapters fighting undead so a cleric would be more useful than a rogue that cannot sneak attack undead) but at least I expect the difference between classes not to be overboard.
And when I said classes I could also add weapons, magic schools, etc... anyone tried to use short swords or bastard swords in BG2?, enchantment and necromancy specialist in NWN2 OC? a slinger halfling in Icewind dale?