At the baseline, you won't have to. Threat per damage is going from 3X to 5X. So just by doing your normal rotation you will be doing more threat.

It's more an issue of survivability. With the model they're proposing they want to add more surv. cooldowns that, I assume, we'll have to hit at high-damage times. This will just add, to me, more complexity if I'm trying to kite, move out of fire, or gather adds.

See, but they balance it by taking out the complexity of having to hold threat. If you notice, very few temporary boss damage buffs are accompanied by a fire or need to move. Looking at Firelands, Shannox doesn't have a damage buff, just the trap, Beth'tilac doesn't buff herself when you need to move (say from meteors upstairs), you don't need to move at all for Baleroc, etc. Rhyolith is an exception, but then again it's a pretty chaotic fight in general and the big add doesn't start off difficult and only starts hitting hard if DPS is slow.

Going back to T11, only Cho'Gall, Nefarian, and Al'Akir have times where you potentially have to move during a high damage attack. (Flame's Orders, Electrocute during Onyxia's sparks, and dodging Al'Akir's tornados respectively.) But they are end bosses so they are supposed to be a tad more challenging. All in all, trust Blizzard's design that high-damage attacks are generally when you can plunker down and fight to survive, while low-damage times are generally when you have to move out of crap.