1) Controllable probably isn't the best term to use, but since UO has no ICD it's effectively
more controllable than BH's proc. During the LK fight, the tank can control the rate that his stacks increase by keeping a ghoul add on him - on Marrowgar, the tank can purposely take several bonestorm hits to increase his stacks quickly. The point is, you cannot control when to gain the Armour buff from BH - it's on a 45 seconds cooldown and has a 25% chance to proc, you simply can't choose to have it proc during 3rd inhale of Festergut or when Sarufang soft-enrages - it's all down to RNG. UO is completely different, and in fact benefits greatly from Festergut's 3rd inhale and Sarufang's soft-enrage (increased haste = more melee hits).
2) The health you gain from UO is added to your current health pool, as health. So whilst one proc may only give you around 500 health (depending on buffs) it's still a form of mitigation; essentially it's knocking off around 500 damage from each melee hit - and that's not something to laugh at in terms of mitigation.
3) Again you're using a situation completely biased towards BH. I can turn around and say, if you're low on health and Sarufang hits you for a wipe, that 2k armour could've mitigated enough damage to give the healers enough time. These extreme situations aren't really the best conclusions to analyse trinkets.
Another thing, you seem to be undervaluing Armour a hell of a lot, here's a snippet regarding Armour as a tanking value:
Armor augments a character's health to a higher number (if the armor is positive), which represents the amount of physical damage a player can withstand. For instance, a character with 5000 health and 50% armor would be able to absorb 10000 damage before dying.
The formula for determining damage absorption is:
DA = (player health) / (1 - (% damage reduction given by armor in decimal form).
For example, a character with 1000 health and 31.24% damage reduction would be able to absorb 1000 / (1 - .3124) = 1454 (rounded) damage.
A character with 1000 health and 99.98 damage reduction would be able to absorb 1000 / (1 -. 9998) = 500,000 damage.
Damage absorption only applies to physical damage.
Full read here:
http://www.wowwiki.com/Armor
4) Your bold statement is simply not true. You can have 400k health, but it's not going to help if you can't mitigate the damage; sooner or later your healers will not be able to keep up with the damage and eventually you'll wipe. On the same token, having 99% mitigation won't help if you only have 1k health. The point is, Stamina and Armor supplement each other, the more Stamina you have the more value Armor becomes. This is called Effective Health, and any serious tank knows the importance of EH, for more info check this link:
http://evilempireguild.org/guides/diminishmath.php