"Argue against game balance?" No!It never fails to amuse me when people argue against game balance by saying it's the GM's job to take an unbalanced game and try to balance it.
Argue against game balance over realism and versatility, especially in a simulationist system.
Game balance is very important, but it should be second to the "feeling" of the game - for example, Black Crusade was not in any stretch of the imagination "balanced", but it was still a great game! GMs could easily separate the game into 2 "power levels", and although it obviously still had some powerbuilds (and omnipotent psykers when played properly), it was still a fun game that perfectly caught the "feel" if the system, and that was easily adjustable to a desired power level (by allowing or disallowing the Archetypes from the supplements and/or Traitor Marines).
However, a game still needs some form of balance: no one player should be better at everything than another, and everyone should have their moment to shine! This works extremely well if the system and GM are explicit about what play-style it is designed for: who cares if your Chosen makes my renegade useless when I built him entirely because the campaign would be based around infiltrating Imperial worlds? Who cares if your PC is a combat god that can kill anything in one turn when we're playing eclipse phase, a game generally involving a lot of loss of gear and investigation?
However, games like d&d 3.0 that had monks being entirely useless at almost every level when compared to good spellcasters or even rogues, you know that there's a serious problem!
People wouldn't grouse about combat balance in the 40K RPGs if the majority of their rules weren't about combat. The combat rules are pretty much on par with those of d&d 3.0 in terms of complexity, with a bit more polish. You can say that dark heresy isn't a game about combat and character building all you want, but the rules beg to differ. I think what you're actually noticing is that for the GM, there are no mechanics to engage with for combat other than using statblocks or making things up as they go. For players, a large portion of the game is stacking up talents and equipment to make fun gameplay combinations.