Do they stack? One's the on-hit debuff from the Felhunter and the other is our debuff curse. They do the exact same thing which is lowering the target's attack power by x amount. Neither are all that impressive against anything that isn't a pet in PvP, but combined in PVE?
At level 70, Curse of Weakness chops off 350 attack power. Tainted Blood at level 70, removes 95 attack power and stacks 5 times, making it a total of 475 attack power?
That's a grand total of 825 attack power out the window. Pretty impressive. Since making opponents in PvP hit your Felhunter is nigh impossible (unless they're actually smart enough to figure out that taking down the demon makes the demonologist vulnerable), I figure it would work best for PvE against elites or such.
If only Tainted Blood still put out threat like in the old days, huh?