First time I've ever heard of it.
Well I assure you they do. I doubt if they would at a Value or Moderate, but they do at the Deluxes sometimes. Than again my understanding is that the new Art of Animation main pool is gated and requires a key to enter and that's a value resort. The time we had to show room keys at WL, they had to see a room key for each person. I had taken my eldest son and GD down there. My son took my GD swimming, while I went into the laundry room to wash clothes. Since I had my room key with me, my son left theirs in the room. A little while later, he came into the laundry room, telling me I had to come out to show my room key to the life guard to prove we were staying there. When I did, the lifeguard said he still had to see my son's and GD's keys, because just because I was staying there did not necessarily mean they were. I had to run up to the room to get their keys. Two families were kicked out of the pool that day that weren't staying there. One left quietly, but the dad of the other family caused a big scene, saying that with the money he was spending for his vacation he could swim where ever he d-mn well pleased, and demanded a manger be called, When the manager showed up he demanded an apology from the life guard for embarrassing his family, instead the manager stood by his lifeguards and the man was told to leave or security would escort them out. The last time I stayed at the YC, I had to show my key just to get into the pool area to take some pictures. I was wearing pants and a hoodie because it was Dec and cold (some people were still swimming however). It was obvious I wasn't going swimming but the CM manning that gate said he still had to see my room key if I wanted to get up close to the pool. I also found out something I didn't know that trip. Swan/Dolphin use colored towels whereas all WDW towels are white. One day it was warmer, so I decided to swim. The people walking in the gate just ahead of me were stopped and not allowed in. The CM made a comment that it wasn't smart to try to sneak in Storm Along Bay using Swan/Dolphin towels, since they were so easily recognizable. lol Not only did those people not have a YC/BC KTTW, but they had the Swan/Dolphin towels peeking out of the top of their beach bag.lol I couldn't believe the CM recognized them like that, just by color.
People have posted that they have asked lifeguards if it's OK if they have guests in the pool and lifeguards allowed it, but it depends on how crowded the pool is and is strictly up to the lifeguards on duty that day.
OT: out of curiosity I asked one of the lifeguards how they knew what resort a person was staying at, since the resort is not listed on the room key. He informed me that each resort has it's own code that is printed on a person's KTTW.