It could be the cases, or the mistreatment by others for sure. Either way if it was a title you owned you could probably send it back to the manufacturer for a replacement (assuming it was only a year or two old). The problem with netflix etc. is they are jus as bad as local rental stores because they seem to have no quality check system on returns unless a customer attaches a complaint on it.
I've always wondered how people do what they do to the discs in the first place to screw them up so bad. Some I've seen look like they wee used as air hockey pucks without a air hockey table.
Or maybe they were doing something like this:
http://www.youtube.com/watch?v=8KvRe4hJyjU
That's why you'll never see 150x speed drives.