I don't particularly see the point, it hardly ever saves any time especially when it gets borked up. There's quite a few mob bunches that are horrible to group up in large herds because of their special abilities:
- Gundrak with the riders on the rhinos that silence. Was healing this one when the tank pulled every single one and then it was touch and go whether it'd be a full wipe or not because of the charging and silencing mucking up my healing. I understand some healers like a challenge in heroics, I've been asked a couple of times on my tank to pull more or tank in more dps gear but I dislike unnecessary deaths for group pulls that took longer to clear because I was unable to heal. The stairs didn't help either so I had to stay in the range of the silences. ;(
- Halls of Lightning... jhjshfjhjfb jkhjkhag hgahhaghgehgah, that sodding last room where tanks try to group pull a bunch of adds then there's death all round as 3 dwarves whirlwind at the same time, or the big iron vrykul get 4 stacks of the poison spear on the healer. The mini gauntlet is sometimes annoying too when the tank gets counterattacked and rooted for 5 seconds or something and they go after something else.
- Utgarde Keep, with the starting mobs - seen a few tanks run in, get unlucky and get stunned a couple times in a row. Not only does this a) mean your avoidance, which in welfare t9 / t10 you grinded solely heroics for and is probably a bit lacklustre anyway, will mean diddly squat it also means b) that the dps still blindly nuking will likely overaggro and c) the healer will pull some of the aggro when the dps use aggro dumps and if the tank is still stunned or trying to grab the aggro back. Hilarity, all at the start of the instance.
Personally I normally try to go as fast as I possibly can but I try and keep an eye on mana, try and keep the pulls fairly sane and it normally doesn't take any longer than any mass herding runs do. Your mileage may vary of course.