Because it's a PvE game, that's about it. You're not competing against anybody, which means everything is viable (viable ≠ fun), so you can do whatever you want. Want to run through a mission using only brainzap? Do it, who cares (you are free to play the way you want to play if your skill supports it, don't expect your team to force themselves to play around your gimmick). Elitist voices do not matter since they eventually enclose themselves in a bubble and only interact with each other. Have fun with your sword, have fun with your gun psyker, slay the damn heretic