...if there are a few cents left on it? If it's illegal to play the credits or to cash them out, in effect that means the machine stays out of use until the casino employees either take out the credits or re-set the machine?

I have seen small credit values left in machines quite often in Las Vegas as people no doubt do what I do - I have played the machines in Las Vegas and if there are literally just a few cents left when I finish that don't cover a spin then I don't usually bother to even try and cash them out and am quite happy for the next player to have a few cents in the machine to add to their deposit.

As others have said in this post, it seems a very short-sighted approach to try and prosecute a player for playing a couple of $$ credit that the casino didn't leave to start with and lose their custom - nearly all players will lose that much in a single visit anyway! Clearly no logic in play here for the casinos