Here's a story of pre-pull regens on tanks.
Was doing the double pull in compass that leads up to the last one (you know, the one with the monkeys that aggro almost immediately after you jump off your cloud, followed by the bi fang and two "dragon weasels" after a bridge).
Grab the first group, run up the bridge, bi fang and co. see me and get angry. Regen ticks before I get in range, all 3 set their sights on the WHM. I Overpower all 3, so the WHM had main enmity for less than a second.
Doesn't matter, the mobs beeline it to the WHM who was a bit of a distance behind. I try to solidify my enmity on the things that are on me, waiting for the runaways to come back. By the time they get back to me, the SAM who was running together with the WHM has almost grabbed one of them for himself (think I heard a Setsugekka go off).
Thankfully the party was made of intelligent people, and the SAM stopped attacking the thing and instead followed us to the top. But there were a few ways that could have gone wrong, which would have made the pull kind of a hassle.
(On another note, both EX roulettes I did today were amazing. This one with the WHM/MCH/SAM was already nice, but the other one with a SCH/BLM/SMN was probably the fastest I've tanked thus far. Daidara didn't even get to widen his lake all the way before he died. Good day to come back after being sick. [I still kinda am, but now I'm at least good enough to remember what enmity is.])