If I'm doing a quest that involves them, I'd bring them along and swap out karlach - mainly because my tav plays the same role and I feel karlach's quests are fewer than the rest
But my main gang is currently me (throw barb), shadowheart (life cleric / tank) laezel (rogue/hunter) and karlach (giant eagle-diving barb cutiepie)
I love karlach and I also love shadowheart & laezel's interactions