Originally posted by Montague
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On loops and rolls and stuff, I haven't had that problem. I've usually had to convince my students that a loop now and again is OK. 
But I do use it as a way to really push controlled straight and level flight with controlled turns in between. I won't let them loop until they show me they can get themselves straight and level and hold a constant altitude around the pattern. The biggest thing I push is being smooth on the sticks, espeically with landing approaches.
Same here, but one of the other instructors has students do loops on their first flight and rolls soon after.
On test flying, I try to get two sets of eyeballs on the plane before a test flight, mine and one of the guys mentioned above. I haven't lost or seen anyone loose a trainer on the test flight at my field. I have grounded a couple as unsafe though. I have my own beat up trainer at the field that I will frequently use if a students trainer is grounded, just so they don't have to go home disappointed with no stick time.
I have only lost one trainer, that was due to a radio problem.
What I was griping about was the lazy fliers who refuse to spend any time on the small details and consider duct tape a structural component of their planes. Someone new with a trainer will listen. Others who have been flying for years may not. I could refuse to test fly their planes, but they would just fly anyway. For safety reasons, I'll take the plane up and keep it on the far side of the field. After everyone at the field has pointed out problems with the plane, and they ignore it, having it fall apart in the air while I fly it is no skin off my nose.