A big reason not to melee, that coll mentioned and I forgot, was that any time you actually would have enough time to melee you should probably be resting. If your MP is fine then you should be doing something else like nuking (which even though nowhere near the power of a BLM your nukes will still outperform your melee). Honestly, I can't even see a good excuse to bring a sword into an XP PT. Staves are where it is at 51+.