T-8 vs: T-12 Fluorescent Ballast

At work this week, we got into a heated discussion about putting a single pin T-12/F96 Fluorescent tube into a fixture that has a T-8 ballast in it. Can someone please tell me in detail , why this should not be done? A T-8 ballast WILL light a T-12 bulb( tube). any help will be appreciated. Mike