I've been running with it macro'd to charge and with the mob at above 75% health, thus triggering the 35% increase to its damage, it amounts to a fair bit of DPS. Debuff slots are unlimited as per blue but there is a limit to what is shown graphically. You're not knocking off a curse of doom.
eh, I like to think I can make all the tools at my disposal work. In the end, it might not be the best option but it does deliver some threat and damage at a relatively low cost and being able to say you used rend on a raid boss is full of awesome.
your hat may be nice, but I have the little white tank top that says Legendary right across my boobs. I win. (or more correctly, H wins)