Having a 10s duration would definitely help with absorbing autos in-between mechanics, and I don't really see any downsides (for busters you can already TBN as soon as the cast begins without any worry, and on big pulls in dungeons you can TBN on cooldown and it always breaks anyway). A shield you have to use proactively is by default more difficult to use than retroactive self-healing used to recover from being hit whenever.
Having to time TBN correctly so the duration doesn't overlap with the boss casting but also some random animations during which no attacks happen can be fun part of skill expression, but it can also be extremely frustrating when you're trying to optimize things only to find out a new weird quirk of the boss, e.g.: learned the hard way that on EX1 after casting Triscourge just before a volcano goes off there is like a 2 second animation during which the boss does nothing, so if you pop TBN right after the cast finishes you won't get hit enough and the shield won't break.
Edit: also can normal raid and dungeon tank busters do more damage? I've lost count how many times I've had TBN + Shadowed Vigil result in TBN not breaking.
