There isn't a single good reason not to.

This isn't singling out warriors - they have it the way every tank should. It should be seamless, to promote tank dps strategies, instead of a painful GCD and combo wipe.

Even healers have their dps stance offGCD.

Warriors even get to keep their abandon/wrath stacks.

It isn't their "niche" it is something all tanks do now and should be able to benefit from.