The way you describe the trash spawn, it doesn't really sound any different from respawns used in any other part of the game... as an instance time limit. A good example of this is Scarlet Monastery, where failure is taking so long that you have to fight to get out of the instance.
And honestly, it's a good gentle mechanic. It's fair for them to put a time limit, and much nicer than just kicking you out of the dungeon if you take too long.
The problem with this is that there is a fundamental difference in expectations between raid instances and 5-man instances.
When you go into a 5-man instance, you expect to clear it, even if it is your first time going to that instance. You may wipe once or twice, but the general expectation is that you will finish that instance. In this case, an instance time limit is a good idea. If you hit the trash respawn, it's a sign that something is wrong, and perhaps it might be a better idea to try again later, maybe with a different group or after gearing up a bit more. If you wipe enough that trash respawns, it is unlikely that your group will successfully complete the instance.
In contrast, in a raid boss fight, you expect to wipe a lot. That the fight is intricate enough that learning and mastering it will take multiple hours. It took us two or three evenings to refine our strategy and master The Lurker Below, and he's generally considered an easy boss in SSC.
Respawns, rather than being a time limit, are interruptions in the learning process. As well, they are interruptions which eat up a lot of valuable time. If you raid from 6-9pm, and you get respawns at 8:30, you may as well call the raid. That means you lost half an hour of learning time, which is a lot for a casual guild.
Respawns are annoying because they make you waste time. Time is the one thing that casuals do not have a lot of. It takes a lot of coordination and administration to gather the people necessary for a raid, and some of that work gets tossed away because Blizzard decided to have respawns.
If we didn't have respawns, we could make better use of the time we have available, we could accomplish more. Probably not a whole lot more, but still. Trash respawns are obvious busy-work. We've already proven that we can defeat them once, why do we have to go through it again? We want to focus on the boss, why not let us focus on the boss?
The point is that respawns are unnecessary. Different guilds have different time constraints. Given that learning bosses is an activity which requires multiple hours, it would be best if guilds could raid to match their own constraints, rather than having to deal with frustrating, artificial constraints as well.
As for the idea that respawns are needed to slow guilds down, I don't think that is true. Respawns don't stop the elite guilds. Multiple guilds have already completed the raid content in WoW. Why slow down the non-elite guilds to an extra degree? Solid and demanding boss fights will keep a damper on progress.
I guess it boils down to the fact that I really don't like "artificial" barriers to content. The content itself should provide the challenge. The challenge in raiding is mastering the boss fights, not clearing the respawned trash that you've already beaten once that evening.
(Which is not to say that there should be no trash. Trash provides little "mini-challenges" and makes the instance feel alive. But once the challenge has been met, that's the end of it, and the trash should stay dead.)