With the recent changes to Rend, it definitely can add up in terms of damage, especially for Arms warriors it is ABSOLUTELY not useless. Arms warriors are gonna be putting it up to proc Overpower Anyways, and as a side-effect, it ends up being a pretty significant DPS chunk because of the Weapon Damage Coefficient that comes from using big 2 handers.
As for tanking, like you've stated it can add a decent damage and threat if used properly, the question is compared to what other available skills, is it worth it to waste a GCD on rend. And I don't have the math to prove/disprove that right now.
Although... i might start toying with charging into battle with a 2hander equipped, rend, weapon swap to sword/board and shield slam O_o lol