Necros and APs would be able to cast sleep at people as often as they want, until it works. Assassins can attempt assassinations as often as they want (especially since when it works, the guy may not be able to tell who did it).

If it fails, they get let off and can try again with no drawbacks. If it works, the guy will often be toast.

I think the wanted system is good as it is. If you've spent a lot of time wanted, it's not the insurmountable problem people think it is. Sometimes you even get more kills because enemies assume that you won't come into town.