There's a few reasons why I pay.
1. I don't (for the most part) want to play long games. I'm very happy sitting down for 30 minutes and playing some arcade game. Street Fighters, defender etc.
2. Some of the games, no I couldn't find. Most of those arcade games.
3. I get to try a game, not just buy it and then be unable to resell it.
4. To me $15 a month didn't seem bad. At that price even if I could find all those games I couldn't buy most of them. I just checked - Planescape sells on amazon for ~$23. That's a month and a half worth of my subscription.
Shrug. To each their own really, but it works for me.
-cpd