There's no such thing as "normal wear and tear" to the physical media. Nothing should ever touch that, and reading and writing data can't "wear out" the magnetic properties of the media. Drives are factory tested to make sure that the coating of magnetic particles is consistent and isn't coming off the platter. However it is possible after time for a defect to become apparent that was not found during testing (or in fact for a head to crash but not immediately fail, though that can't be common. That's when you develop bad sectors. Once you develop one bad sector, chances are good that the defect is not limited to just that one sector, so you're likely to start getting more and more. It's safer to just replace the whole thing right away.
Much older drives I think were more likely to have bad sectors, and it wasn't quite as imperative to replace the drive. One could assume that once it was marked bad and no longer used, it was probably just a defect in that sector and everything else would usually be fine.
Think of it this way: would you continue using a floppy disk for important data if a format found a few bad blocks?
Format is slow as can be no matter what you're doing. If you happen to have any other utilities handy (like PartitionMagic) they can usually scan a drive faster and mark them bad faster.