Had a magnetic one. Picked up a nice sized metal shaving from the garage, it didn't do too much damage, but I caught it quick and my bike was far from pristine. If I had a mint bike, I don't think I'd be able to use one after my experience. With a bike that I have to ride, I'd probably risk it for the convenience factor. water bladder in a mylar freezer bag(fill half full and freeze the night before, fill with water & place in freezer bag = 10 hrs of cold water....buy drinks, but sip from the bladder), snacks, phone & charger hooked to 12v, map, some basic tools, you can fit a lot in one of those bags.
Just popped in my head, the should make little metal "covers" for the magnets when not in use. Would prevent most stuff sticking and would also force the rider to inspect before installing, which would prevent a lot of the issue.