It won't appear as a weapon because it already technically exists as the form of the level 3 melee Limit Break - so that would just be confusing to have it as an actual weapon. Not to mention the Buster Sword is a two-handed sword, not a single handed one like GLA/PLD wields (but DRK would be suitable naturally).

Also the Cloud outfit is his Advent Children costume, which, considering he never used the Buster Sword in FFVIIAC, instead using his new 'six-in-one' Fusion Sword, I still doubt we'll see it as an actual weapon.