I think I'm a fan of the idea that only a few key units should have the ability to gain vet. Mainly ACUs, support commanders, and experimentals in my mind. But any unit that where vet can enhance gameplay is fine with me.
I don't know how much of a performance hit the vet system makes, but I'd say it's only worth changing this if there is something to be gained. If we can save some cpu cycles by removing vet from all of my ASFs or T1 spam, I'd be all for it.