When you're working towards the Warclaw, I recommend you use a shortbow thief to keep up with the zerg.
You can switch back to the class you like playing after you have the Warclaw, but trying to keep up on, say, a Scourge is ... unlikely, fatal, and not recommended.
[Tip ends (as all tips do)]
Thief OP? Better nerf Scourge ... again.