They have been sitting in a climate controlled room all that time.
Not that I know of. It's just an observation that our group of about 15 people agreed on. These were movies that were used a lot (pr0n) and were visibly getting worse (banding, tracking, wear, etc.), not just sitting on a shelf. We would also share them with each other through the mail and use the GoVideo dubbing decks that we all had to make copies. This was for alt.binaries.movies.erotica and associated newsgroups. Probably TMI :)
Anyway, it all depends on the storage - the best advice I ever heard was "store your things where you live" (in other words most of us don't live in the attic or the basement).
Honestly, I think in a single act I nearly destroyed my youngest son's interest in the series by having him watch the first one with me. I haven't seen those movies in years, but it really reminded me how big Star Trek was back then to be able make such bold assumptions of the audience on so many levels.
You know though, all this talk - I think I'm going to dig up another and watch it tonight. There's actually a couple I still haven't seen.