Here's the problem - games start on a regular basis, like 8am, noon, 4 o'clock, etc. So, let's say you're wanting to play some game and you've got your generic tickets in hand. You find the location of the game, go and stand around, and sit at the table. The GM shows up, and then asks who has tickets to the event. Those that actually registered for that specific event are given precedence, while those with generics are taken on a first come, first serve basis if there are openings. Sometimes you can get into a game this way, but sometimes you can not. So, if you're standing there at 4:00 (or whenever) and the GM says, sorry, the game is full, I'm not taking any more generics, you're left standing there in the cold. And, furthermore, it's now about 4:15 and not only did you not get into that game, every other game that was scheduled to start at 4:00 has already started, and you're screwed out of being able to play in those as well, so you have to wait until the next time slot, which is probably 4 hours away.