Scizor already has a great movepool. Sure, it's attack moves are somewhat limited, but many more good Pokemon have more limited attack choices, and we love them just fine. Scizor has a lot of great support moves at its disposal.
Scizor's Speed is good enough IMO. It goes down a little bit from Scyther, but it really doesn't matter much. After one Agility, it outspeeds most everything anyway.