After some thought, I think I might hate trash respawns because they are disproportionately affecting my guild.
My guild raids 3 days a week, for three hours a night, from 6:30 to 9:30. We can go longer, but there are people who have to leave at 9:30, so we have to do swap-outs.
Most trash now respawns on a 2-hour timer. That means that trash respawns in our third hour of raiding. So we're always faced with the choice of reclearing trash and extending the raid to get more attempts in; or calling the raid early. We very rarely get to end the raid on time.
If trash respawned on a 4-hour timer, we would almost never see it respawn, and thus I would not really care about respawns. Or if we raided for four hours a night, it would be an inconvience, but a relatively minor one as we could reclear and get in several more attempts before ending on time.
Is a three hour raid window unusual? I don't think so. I think it's a very natural slice of time for a more casual guild to schedule a raid. Unfortunately, it just happens to mesh badly with 2-hour trash respawns.