The top itself is subject to stress and strain that, in the worse cases, may cause it to crack. I was looking over at CorvetteForum for clear top issues, and, came upon this thread, which includes a couple of anecdotes about clear tops cracking. These are on the newest generation Corvette, which also had the problem with the colored roof panels that weren't glued on well, and would fly off.

I know driving around in my 1994 Camaro equipped with T-tops, that the tops moved around quite a bit while on the road. When I put my finger on the separation line between the T-top and the windshield frame, it was like touching the San Andreas fault - the tops wiggled quite a bit on bumpy roads. They never cracked, though, they were glass.