Sure, just plain paper will work, too, but the feel of the laminated flats is much better, and you won't have to worry about them being crinkled in transit. The idea comes from campaigns like those for Roswell and Jericho, where fans sent the powers that be tabasco sauce and peanuts, respectively. The flats are a solid, physical thing. Laminating them makes them feel like a little gift, instead of just a paper doll.
I'm happily picturing our flats decorating at least a few Disney employees' desks as more and more of them are sent in. They're just so durn cute!