- That side wasn't necessarily quality tested. Since disks, like CPUs today, were binned, you could often use the "bad" side anyway, as the failure rates went down on the production runs.
- That side didn't have a dust filter, or whatever you call those little felt pads.
- If it DID have a dust filter, it might have been in a position to catch the dust on a true double-sided drive, which would be spinning in the opposite direction, and so would end up being useless to you.
- Or, worse, if the pad was napped, like velour, spinning in the opposite direction would actually release all the dust that had been captured. That would be bad.
Or at least this was the lore that we passed around on BBS's at the time. It could have been every bit as wrong as the truism "When your computer crashes, try wrapping your disk drive in tinfoil" (which was in fact true if your problem was RF from the TV hitting the poorly-shielded-and-filtered serial cable, but otherwise either useless or a good way to overheat your drive).
Boy, it's been a long time.
I think in the early days of floppies the quality of the media and the drives was much better.
After buying a 5.25" drive (from ebay), acquiring a PC (from the dump, literally) that was old enough to have a drive controller that could talk to the drive, and tracking down old software for reading Apple disks on a PC, I found that the bits had just faded away and all the disks were unreadable.
Time and money (and luck) can't beat entropy. It wasn't even much fun.
[1] http://ascii.textfiles.com/archives/3191