When does one unzip the window?

Some can't be unzipped on more modern cars. Like my 2008 Chrysler Crossfire. It's a soft window never been a problem. My 70 Challenger window a hard glass zipped window (plexiglass) pulls a bit harder on the top. The window will not bend. I've always unzipped them if I could and layded a towel over them to avoid a scratch. Probably cause I'm to cheap to replace on if , by chance it got nicked.