You got me there on the physical dmg bubble XD
I will however present you an conter-argument:
500Stam is around what, ~8k hp ? compared to the i'd say minimum of 400k raid tanks have it's a 2% increase.
on the other hand IP trinket procs every min, and 56% of an average boss white melee hit is roughtly 100k form the handful of WoL logs with paladin tanks i've seen. Sometimes a bit more often a bit less, in hc is pretty much always a bit above 100k.
Lets go with 100k it's a nice round number.
So... we have a 400k+ tank eating 100k white hits, there's a pretty decent chance of having a 56k bubble every minute, which is fully absorbed by the following hit. Meaning around 933 damage absorbed per second.
Actually now that i think about it, 56k abs on100k hit is kind like having one critical block every minute or a nice buffed up free WoG per minute.
Heck, lets say it doesn't proc that often and nerf that value by 15% and get 800 per second. 800absorb/s vs +8k health. Seems obvious to me that the stam loss is no big deal compared to the trinket proc.
I'll stand by what i said, it's looking like a pretty sweet trinket @lvl90. Sure it's no mirror, eventually you'll want to remplace it, but it'll last you at the very least until you are finished with normal mode t14 (so only remplace it with 489 or 502 trinkets)