1. Trash doesn't live long enough to be worth marking, most of the time.
2. DPS NEVER follow marks. Yeah, if they pull aggro, it's their fault, but the healer still blames me for letting the DPS get the hate. DPS gets a little thrill of pride, "Oooh I can tank!" and "Oooh I managed to out-DPS our tank's aggro generation! Hahahaha! I'm sooo awesome!"
I shouldn't say never. It's always a case of one of the DPS follows the marks, and the other one doesn't. The number of times that this is not the case is a single digit percentage.
It's so much easier to let the DPS attack what they want to and hold the hate off of them, because they're gonna do what they want anyway.