Do they give you a proper ticket when you get there ?
Its probably not at all relevant here, but I've been to a couple of gigs where they now don't issue you tickets at all, you just present the credit card you paid with, its scanned to check and then they let you in.
Its obviously to stop people selling tickets on and touting, wouldn't surprise me if it becomes more popular.
Bit rubbish if your credit card expires or you have it lost or stolen, though. I sold tickets once and sent them my card as it had expired nothing bad happened.