Having a powerful car like the Z you can't help but worry about some knucklehead getting a little too enthusiastic with the throttle and causing an accident, or screwing up a shift and messing up your tranny. Friends and family who want drives are the worst. Most people who own slow or semi-fast cars aren't used to the power of a stage 3+ TT.
I don't mind too much of a mechanic has to test drive the Z to fix a problem. But if they fuck up my car they're getting a lead pipe to the cranium.