I can't speak for everyone who collects autographs, but for me the value in getting an autograph is that a) I get to meet someone famous and b) walk away with a tangible memento of the encounter. It's really no different than people wanting a photo w/ a celebrity, or collecting any number of things (Disneyanna, sports memorabilia, parking tickets, etc).
That being said, to me the allure of Sid's was always the props, costumes, old movie posters and personal items from celebrity estates and not the autographs. All of that stuff was truly unique, and they had a wide selection of price points. Even as a poor college student, I was able to buy some items from Flubber, Armageddon and Alien Resurrection.
Sadly, that's all been long gone, so I don't really care what they replace it with.