Considering most of the bosses in the current tier of content do just that (either by having you swap aggro on a single major target between the 2 tanks or by swapping between 2 bosses that specialize in different types of damage dealt), I'd have to wonder why you think they're "bad" when they're actually a better to way to leverage increased survivability when you need it rather than having it up at all times, including those when it's not even useful, which amount to 40-50% of the time on the said bosses.
I've always found that I get better performance out of the tanking use relics than I do out of the passive relics on all 3 of my tanks, but I actually *use* them. If you're having a problem with any of the use relics, it's not a problem with the relic itself but the player's reticence to actually use said relic. It's like complaining that a survivability CD isn't strong enough because you don't use it often enough.
Only 2 paragraphs? Kitru, are you feeling okay!?