We started going to WDW in the early 90s. Back then they used to advertise that if you stayed on site you could use all the facilitites at other hotels. As more moderates were introduced along with the All Stars this policy changed. Now, you can use the pools at the hotel you are staying at. In the case of YC/BC you can use quiet pool at either one if you are staying there. And, this is also true at the PO-FQ and Riverside. It is only in those cases where resorts are considered to be "sister" resorts that you can pool hop. The main reason is that places like YC/BC were being inundated with people from other hotels. Those hotel guests that were staying there were having a problem finding a place to swim. In many ways I can understand why they changed the policy.
In the slow season, though, when we typically go, I haven't seen them check at IDs at many of the pools. I think this is the case during the peak seasons such as summer, Christmas, Easter weekend, Presidents weekend, etc. Then I have seen them ask people to leave or go back and get their room key to verify that they are staying at the hotel.