They are nice to have. You can clean the glass without getting wet. Only downside: if you get sand caught it can scratch the tank. Also don't use glass pads on acrylic tanks as it will scratch. Acrylic cleaner on glass will work though, but not as good as glass pads.
I use them. Never had a prob with glass being scratched. A note worth mentioning, I have Flourite and Flora Max in my tanks. The magnet will attract pieces from time to time if I get to close to the substrate. I'm just really careful when using it.
