Quote Originally Posted by Mo-Nercy
I say, if it's in the friggin' game, you're allowed to use it.
This is absolutely true in most cases. In the case of RPGs, most of the 'cheap' items take a heck of a long time to get. Anyone who's willing to put in the time and effort to get KotR deserves to use it whenever they like, if you ask me. Otherwise you might as well start criticising people for levelling up.

On the other hand, there are certain rare occurences when a certain character or weapon can seriously upset the balance of a game, and in those instances those characters can be quite rightly banned from serious play. Other times, stages are banned for presenting an unfair advantage to certain characters (Hyrule Temple + Fox = Suckiness). I generally find it's a good idea to agree on a set of banned characters/stages before playing someone even semi-seriously. That way, there's no way your opponent can blame you for kicking their butt.