It depends on the nature of the defect whether a floppy drive will be killed or not. Most bad disks are caused by degradation of the surface, and at worst that just means the surface coating sticks to the heads, making them useless. However cleaning them should solve that one. To actually break a drive there must've been something else wrong with the disk - a ridge in the media itself which knocked the head out of alignment or something.
Don't worry, at this stage it's always wise to have a spare drive nearby :-)
