The Horrible Effect of Parry

[Mavfin]

Once you have Sunders built up, not Devastating on every GCD you're not Revenging or Shield Slamming sounds like quite a waste to me.

Heroic Strike and Shield Block operate on their own cooldowns, and if you're not Sundering, you'll have plenty of GCDs to spare for Devastate. It's more threat.

And after all, Devastate doesn't consume a white hit.

Of course, this is true, but, it's as you say: When tanking, I only use Devastate when I don't have anything else to use the GCD for, other than making sure I use it enough to keep the sunder stack up.