As long as you're taking the time to notify the hotel that you do not need the ADA room, I see no issue with it. It's the hotel's responsibility and/or choice to make that room available to someone who needs it.

It's the same thing when booking a cruiseship cabin. You can book a person who does not need a handicapped accessible cabin into one (sorry, but cruiseships don't tend to use the term ADA) however, the cruiseline makes it clear that if someone with a need comes along, your client will be bumped out of that cabin into any other one in that same category or higher. There's nothing ethically wrong with it -- either the cabin will sit empty, be occupied by someone who needs it, or be occupied by someone who doesn't need it in lieu of sitting empty.

I see no difference in the hotel-scenario. Now, if you booked the ADA room under false pretenses, that would be a different scenario. The OP didn't indicate that he/she was being underhanded about it. He/she is not getting the room at the expense of someone with a real need -- he or she is getting it in lieu of it sitting empty. No moral dilemma there.