Here's a question I've been wondering about recently: How adversely does haste affect your survival from additional parry hastes? Basically with all the haste on my new TOC gear I'm wondering if expertise just becomes even more important as a mitigation stat.
One of the very first posts I ever wrote on this blog was about expertise and survival. It's interesting to go back and read that too. The conclusions are sound (stack expertise over avoidance) but it somewhat misses the overall idea, which is whether or not it's actually that important, and whether or not you can circumvent it with simply having more stamina.
Note that it's much less relevant when you can't be crushed. Having two crushing blows in a 40% speed window + a special was much scarier than two normal hits + a special. But that's not what this question is about; this is about how much haste hurts you with parrying.
here's some rough analysis of how haste will affect your survival.
In 240 seconds you have 160 GCDs and 40 cycles of 6 second mangle/x/x/FF. Of those, 3 can be parried. You will also normally have 100 mauls.
So in 240 seconds you have 120 parryable normal attacks (1/4 are faerie fire) and 100 mauls. Let's assume you're soft-capped for expertise, meaning you have 26 expertise. This means that you will have 8.5% parries overall, or 18.7 total parries in the fight.
If you have 1% haste, your attack speed drops from 2.4 to 99% of 2.4, or 2.37. That gives 101 mauls instead of 100, and now you have 221 parryable attacks. You're likely to get 18.78 parries.
How about 10% haste? Your attack speed is 10% faster than normal, or 2.18, and you are expected to have 110 mauls in the same time. Your total parries?
So 10% haste means you will, on average, get parried .8 more times than you would have before if you have decent but not stellar expertise in a 4-minute fight.
Can that kill you? Yes, it can. It's not nearly as scary as it was before, but parries can certainly kill you on bosses that parry-haste. Thorim is about the only one I know of that actually has killed anyone recently, but it's theoretically possible.
Should it kill you? Well, even with most hard hitting bosses melee attacks aren't what is going to obliterate you. Thorim is one of the only cases I can think of where it can; most of his damage tends to be scary melee towards the end. Hodir has frozen blows, Vezax isn't really scary because of that at all, Gormok's scary because of impales + melee, Anub is scary because of freezing slash + swarm + melee. Now, if you get unlucky and get two very fast melee hits + a special, you can drop just like that. It's possible.
But I've not seen it happen to me yet.
Why is this the case?
Well, the main reason is the damage profile. Healers have to heal you like they did back in Brutallus days due to how much damage is coming in. They must keep you topped off if at all possible. Because of this, small bursts tend to be wiped out by healing. That's not always the case, but it often is.
Note that before Gormok's parryhaste flag was turned off, he absolutely could parry haste and he absolutely could wipe the floor with someone, especially if they got parried twice in a row. Three quick melee hits within a second is both huge amounts of damage and unexpected for healers to deal with.
Anyway, hope that helps. The long and short of it is that for a pure survival build you should max out expertise if possible and minimize haste if you haven't. If you have, of course, you can safely have as much haste as you want.