The readers aren't very effective at stopping admission cheats. You can choose not to register your fingertip, and to use photo ID for admission instead (I'm thinking of having a random piece of photo identification made with the words "OFFICIAL BOGUS SECURITY IDENTIFICATION FOR HOTELS, THEME PARKS AND OTHER JUNIOR G-MEN" printed on it). So it would be very easy to share your pass: the person named on the pass enters with his ID, and the person with whom he's sharing the card uses a fingertip -- you could visit with your sister's family and half of you could use the tickets in the morning while the other half hung around the pool and relaxed, then switch at lunch: the morning crew uses fingertip, the afternoon uses ID.
What these readers are effective at is conditioning kids to accept surveillance and routine searches and identity checks without particularized suspcion. One morning at Epcot Center, as we offered our ID to the castmember at the turnstile and began to argue (again -- they're very poorly trained on this point) that we could indeed opt to show ID instead of being printed, a small boy behind us chirped up, "No you have to be fingerprinted! Everybody has to be fingerprinted!"