Of course the game is not "unplayable" just because people can cheat. Just agreeing not to do it works fine.
I play board games with my friends all the time; we don't immediately throw the game in the trash just because it's possible for whoever is sitting nearest the bank to covertly take some of the cash.
I wouldn't play Civ4 PBEM with people I don't trust, but that's true regardless of whether any particular bug gets fixed. There will always be ways to cheat, at least in PBEM.