Don't modern drives replace a bad sector with a good spare one internally when the bad sector is next written? The days of entering bad block numbers and also writing them on the hard drive's "bad blocks" label have gone, haven't they?
If I've misunderstood, then perhaps the Rationale needs expanding.
It also possibly puts the harddisk under more stress, possibly increasing the chances of drive failure.
Furthermore, passing a bad block test doesn't mean the drive is good, it might start failing a few hours after.
That being said, make it an option, but put a warning. It's important to let people do what they want, when they want, and if they are using a refurbished harddisk, realistically, I'd want to run a bad block test myself..