I have seen people questioned at the Polynesian and at the Boardwalk. There is signage at all resort pools stating that the pool is for the registered guests of that resort only.
Can you get away with it? Maybe. But, if anyone even suspects that someone is at the pool under false pretenses, they will tell someone so it can be acted upon. I'm sure that's what happened at both the pools I mentioned...it wasn't overly crowded either time. But, because those people, who were questioned by the lifeguard, couldn't show any resort ID they had to pack up and leave. They were not happy campers.
My suggestion? Pay to stay at a resort that offers the amenities you want. If a hugely themed pool is not important to you, then you can make do with a moderate resort. But, if you want to have a 'better' pool slide and more extensive theming, then you will have to pay for a deluxe. Those 'extra' amenities are what you are paying for when you pay those high room rates. Fewer amenities? Lower prices. It hardly seems fair to have someone stay at a value resort but feel that they can use the pool at the Polynesian (for instance). But, that's just my personal opinion.