Wits as a battle stat makes sense, because one of the most important parts to combat is out thinking your opponent. This is not intelligence, wits is very suitable as a combat stat.
I'd love to see some tactical skills play off of wits, such as chance to trip/dodge/disarm/counter-attack/blocking...
As for backstabs, the idea behind this is that we don't anticipate or protect ourselves from behind as well, and someone with small blades can hit some more critical areas of our biology a lot easier.
Initiative is a good call to be linked to wits, crit is also suitable but useless, and perception is critical as an out of combat stat.
The problem is that, choosing a party member to suffer in combat, just so that you have someone with perception is not very fun design.
Backstabbing should provide a damage bonus based on the wits stat (rather than ignoring it).
