As Tint pointed out, BP is damn good on large trash pulls. The larger the trash pull, the better BP gets. The amount of Abysmal Drains you can get off is pretty dependent on the MP return of BP, and you can even DA one or two of them to help out your healer a bit.

I want to say buff it to be more effective against single targets, but that's what Blood Weapon is for. These two abilities are nearly polar opposites designed to complement the stance you choose to be in. Against bosses, you're going to be in DPS stance 95% of the time unless something goes wrong. If BP is missing something, it's probably a slight increase to defense while it is active.

Don't get me wrong though. I really like the idea of giving DRK spikes that causes damage to their assailants. But I think it would function better a new ability than an upgrade to BP. I believe BP is fine as is.