I think it's abusing a glitch. Some people don't think it's fair, I don't mind it and used it a lot in gen 2 and 3. A few times in gen 4 too.
If I put a clone up for trading, I always tell the other party about it, just in case they see it as cheating. So they won't feel scammed if they do the trade since they know it before they trade their pokemon.