There's a few things you can do. Most of the time I don't have runners on adds I've already had tanking, though there are rare cases (bladestorm being one) where I'll have to taunt it back.
If an extra pack is pulled, no big deal, I pestilence it. Failing that, I'll IT one, DC another, and taunt a 3rd, leaving DG as a possible 4th option, but by that point pestilence should be up.
I dont think there have been many times where I've lost aggro (even on AoE), and even fewer times where I've not been able to pick every mob up.
As far as single target threat in a raid environment, I don't think there's too much of a concern there, though I'll have to keep a closer eye on it. When I MT'd a few bosses in 10 man naxx I had significant (30% or more) threat leads.
Edit: I am Unholy though, and have been for most of the time. I can understand some AoE/picking up adds being more difficult for frost (UB works really well for AoE), but I think it's doable.
Luck is what happens when preparation meets opportunity. -Seneca
Everyone marvels at a square egg, but only the chicken understands the PAIN.