There is a very common misunderstanding when it comes to health increases for tanks. It's often said that Stamina/HP are only useful for surviving getting one/two-shot, and outside of that they don't do anything for survivability. Or, for example, if a talent increases your health by 10%, and you never drop below 10% in a fight, some will say that talent did nothing for you. This causes many people to undervalue health increases, such as Stamina flasks, Stamina on gear, talents that provide Health/Stamina, etc. In short, extra health does the exact same thing that damage reduction does, except that it has to be healed. Now you may be thinking that's obvious, and that's the reason you don't value extra health, but bear with me.
Let's look at this from a conceptual point of view first. You have to pick between two talents. One provides damage reduction, and the other provides a health increase. You take damage, and with the DR talent you're left with 80% HP. With the HP talent, you take that same damage and you're also left with 80% HP. For surviving that particular hit, both talents did the exact same thing. Again, the difference being that with the health increase that extra health has to be healed up. Damage reduction makes your health bar move less, and so does extra health.
Another way to look this is to imagine that all healing you receive is percentage based. If that were the case, there would be no difference between a DR increase and a health increase assuming they both provided the same amount of EHP (effective health pool/effective health points).
Damage reduction is good because it reduces the chance that you die. Health does the same. Now, from the example above, obviously the damage reduction talent is better; it had the same effect in terms of surviving the hit, and you don't need the extra healing that you'd need with the health talent. You don't base your overall mitigation on just barely surviving damage you are sure you are going to take. You base your mitigation on making sure there is no realistic risk of dying. Even if you didn't drop below that 10% health, you want that safety net there just in case something goes wrong, either with you or with another raid member. How big your safety net needs to be is up to you - just remember that without one, you die. And most importantly, for the purposes of this discussion, remember that that safety net is not just stamina or extra health, it is all forms of damage reduction of any kind.
Taking that concept a step further, say the two talents choices are a flat 10% damage reduction, and a 20% health increase. At first glance, I think most people would choose the 10% DR. This is where things get a bit tricky because how powerful healing is relative to your health pool has an effect on how useful that extra health is in comparison to the DR. Something that some of you may be familiar with is the fact that you can express DR in terms of EHP. The formula for that is:
HP / (1 - DR) = EHP
That gets you your total EHP. For the increase to EHP, you simply subtract out your starting HP:
HP / (1 - DR) - HP = EHP
Let's say you have 100 HP. So, with the 10% DR, your EHP would be (100 / (1 - 0.1)) ~111.11. Or an ~11.11% HP increase, less than the health talent is providing you. You can also do the opposite. That is, express health increases in terms of DR. The formula for that is:
healthIncrease / (health + healthIncrease) = Corresponding DR
For our health talent: 20 / (20 + 100) = ~.16, or, ~16.66% DR.
If healing that extra health isn't an issue, the health talent is objectively better for survivability. It makes your health bar move less and reduces the impact that damage has on your overall state. Generally speaking, healing that extra health in a real raid environment isn't an issue. However, that depends on a lot of factors. It depends on your healers, the damage profile of the enemy, etc. That's why it's often so difficult to compare a flat DR to an EHP increase. Point being, health has a direct and observable effect on survivability.
As I've said before, health increases have their disadvantages. There's a reason no one stacks health. Your "initial" survivability might be great, but you're a burden to heal and you'll likely reduce your raid's overall success because the healers have to give you so much attention. There's definitely a balance to be had.
Let's talk Stamina for a minute. Stamina is extremely efficient at increasing your EHP compared to other stats. 1 Stamina grants you 20 HP. An easy comparison is Versatility, since it provides flat damage reduction. 170 Versatility grants you 1% DR. To make them comparable, I'll express the DR from Vers in terms of EHP increase. A current example is choosing between a Fish Feast and a "normal" Feast. The fish feast provides 85 Vers, or 0.5% DR. The normal feast provides 22 Stamina. You have 50,000 health and 8% (DR) Vers.
Assuming those are the only two things that affect your EHP, your baseline EHP is 50000 / (1 - 0.08) = ~54,348. That 22 Stamina increases your EHP to 54,348 + 22 * 20 = 54,788, or a 0.81% increase. With that extra 0.5% Vers, your EHP is 50000 / (1 - 0.085) = ~54,645, or a 0.55% increase. Now obviously neither of these feasts is providing much of an EHP increase, but, that 22 Stamina is providing more EHP than 85 Vers. Your healer definitely isn't going to notice that, so from a pure survivability standpoint the Stamina feast is better.
It is worth noting that because Stamina does not increase your health by a percentage but rather a flat amount, as your health increases, a point of Stamina provides less and less EHP relative to your health pool. Percent DR is the opposite. For example, for each point of Stamina you get, subsequent points do less relatively speaking. For each point of Versatility you get, subsequent points provide more EHP.
As you can see, it takes quite a lot of health for Stamina increases to be worse at increasing your EHP per point compared to Vers. Note that in this graph the additional Versatility is not increasing (and neither is the Stamina), which is why Vers has a slope of zero. Regardless, per point, Stamina is best for increasing EHP for quite a while. And even when it turns out it's not, it still of course has value even though it may not be as efficient as it was.
Anyway, the point of this article is to simply highlight the fact that health is much more useful than just "surviving one-shots". I hope this at least changes the way some people look at health and its value for survivability.