I cannot find any good uses for Reave. From what I understand, it absorbs a little health and weakens them (and while they are weakened, I get a little bit of extra defense).
But it does not work on many things it seems, like Geth Prime. Normally when it hits, it gives me a glowing aura that represents my extra defense. But when I use it on say Geth Prime, nothing happens.
Best Answer
It's very good for spamming biotic detonations with another biotic, for several reasons:
It's also good on its own against organic targets, but less useful against synthetics, since you only get the extra defense when it's used against organics.