There's currently many complaints about the rate of trash respawns in the endgame. So far, it hasn't really been too severe in Karazhan yet, in my experience, but it looks to get a lot worse.
But is it really necessary to have trash respawn in an instance?
(Note that I am not talking about fights like the BWL Suppression Room, where the trash is part of the boss fight. Such trash should respawn if the fight was lost.)
I like trash mobs. It's a necessary part of the instance, and can lead to some interesting fights. Small twists without being a full on boss fight. An instance that was all bosses and no trash just wouldn't feel right to me.
But I don't really see the purpose of having trash respawn. I suppose it slows down progression, but unless the trash is truely excessive, I don't really think that it really affects the cutting edge guilds.
Realistically, trash respawns just make life hard on the lower tier guilds, the ones which aren't as good as the high end guilds. And I'm not sure that slowing down these guilds with trash is really necessary. The boss fight will block them from progressing.
I've disliked respawning trash ever since Scarlet Monastery. If I had a dollar for all the times I've gone through the instance, killed the bosses and been killed while running out, I'd be rich. And that's just silly in my view. Would it have hurt anything if the trash had just stayed dead?
If I recall correctly, trash in Blackwing Lair stayed dead once killed. (Well, sometimes you had to kill the entire group, if the trash was linked, which is an acceptable limitation.) And Blackwing Lair was a good raid dungeon. Did the fact that trash stayed dead make it a weaker dungeon? I'd argue that it made it one of the better dungeons.
Hard trash mobs, interesting trash mobs, linked groups of trash, that's all fine. But once the trash has been dealt with, it should stay dead.