Yeah I was just explaining what worked best for my purposes. I don't see any delay in startup when Game Center is authenticated, so not sure about that. As far as using those accounts goes, I just know that people don't always like having to create a new account in each game and I figure Game Center is already widely used. I probably would've used strictly Game Center if I didn't want global leaderboards for every user created level in my game.
Parse does have account creation and things like that if that's the route you want to go. They even have templates that you can customize for the account creation/login screen.
Oh, and as far as Game Center hacking is concerned, it's true that you can now moderate things, but I'm not sure how easy it is to stay ahead of hackers that way. I need to look into it more.