It's in the same class as Ravenous Rats I think. The discard effect is weaker but is STILL a -1 card to them - a card they haven't draw - and you get a more useful creature tied in with it.
I imagine a deck of 4 Black Zubera, 4 Ravenous Rats, 4 Chittering Rats, 4 Phyrexian Ragers... there's a lot of card advantage wrapped up in those little abilities, nibbling away at what choices your opponent has available to him.
I'd be surprised if they make it into tournament decks, but they're servicable enough.