I've been placed in these rooms a few times, but I'd much prefer a standard room. I don't like them because the shower floor is tile, and there usually isn't a barrier between the shower and the rest of the room (except for the curtain), so water tends to make it onto the floor of the rest of the bathroom.

The only "rules" I know of for these type of rooms is the property is not allowed to charge more for these rooms than a non-ADA room.

I wouldn't feel guilty about staying in one of these rooms. If the hotel absolutely needs it, I'm sure they'd be willing to upgrade you to another room. This isn't like the disabled seats on a bus or handicapped parking spots.