My fil's sister was going to WDW last year and knowing what a WDW nut I am, we were talking about her upcoming trip. Well, she said she couldn't wait to try out the new pools. When I told her that pool-hopping wasn't allowed, she was quite taken aback. Told me that she had always pool-hopped. That when they went to a park for the day, they would pack a big bag with their suits in it and leave it in a locker and then take a bus to one of the resorts for a quick swim. They had never had a problem. When I asked when she had been there last I was told 'oh, about 12 years ago.!!' She was surprised to hear the change. When I explained the reasons she did say it made sense.
So....do people do this? Yep. But, be advised that I have seen people asked for resort IDs and then they have had to pack up and leave. I don't understand why you don't swim where you stay. As a DVC owner I have the option to pool hop but I don't think it would be worth the hassle.