Since Oklahoma City, maybe it has something to do with car bombers. Or maybe it's a deterrent for people who might come in and steal from people's rooms. This, by the way, is one reason that it is always a pain to get in and out of shopping mall parking lots; they are designed to slow down shoplifters and other thieves. Criminals see gates like this, and they usually decide to stay away. Easier pickings lie elsewhere.
I also wonder if everyone needs to stop so that a security camera can take their picture, including of the license plate. If, for example, rooms were robbed at the Yacht Club on a Wednesday, Disney security could check the license numbers to see if the same car happened to go into the Contemporary on the preceeding Monday when rooms were robbed there.
Overall, I go through the gates only a few times a day, and they don't bother me very much at all. Spending 5 seconds at a gate, at most, causes a delay of a minute or two per day, total. Compare that to all the lines at the attractions, the bottlenecks at each park's bag check, or the delays in getting food just about everywhere. The extra security is worth it in my opinion.
