Below are a handful of reasons RDM shouldn't melee, especially at later levels.

1) Every hit that a melee lands feeds the mob TP. This causes the mob do do extra strong TP moves. If you are hitting the mob for 2-5 damage while everyone else is hitting for 15-30 damage all you are doing is feeding the mob TP while not really doing enough damage. In XP, even with enspells, you can't really do enough damage to justify the TP you are giving the mob.

2) If you want your melee to begin to even approach sub-par you'll need to use gear that will help with that (STR, Atk, Acc). If you are focusing your gear on these things you aren't focusing on MND, INT and MP. You can't really get away with macroing the gear around either and the cost is beyond the reward. Most people aren't going to spend extra time to make money for a melee set and an enfeeble set.

3) By the time you can use elemental staves you have no excuse to melee. You should be macroing your staves (and other gears) to help you land enfeebles, as this is your primary job next to Refreshing and Dispelling. RDM has no natural staff skill so you won't be able to hit anything with your staves really and even if you did you'd be macroing different ones in so regularly that you wouldn't be able to keep TP for yourself which makes your damage even more pointless.

4) There simply isn't time, as you level higher, to do anything but your job. You need to be enfeebling, keeping up a refresh cycle, dispelling as the mob buffs itself, sleeping links when the PT gets them, and throwing out back up or main cures. I don't care how good some people think they are at RDM but the truth is, you can't find time to melee while trying to juggle all of the things that your job needs to juggle, regardless of the fact that your melee is crap at best.