As other people have said, if you collect from
older shows, sometimes the choice is a cel in less
than pristine condition, or no cel at all. And for
a lot of collectors, no cel at all is simply not
an option.

Have you ever pointed at the screen and said to
yourself "I want that cel."? When such a cel is
offered to you, are you strong enough to say
"nope, gotta give up that dream, there's line
fading."? I'm not.

As for stuck to drawings, I've had a great deal of
luck unsticking cels through various methods
detailed in the archives here. I never assume that
a stuck cel is stuck forever.