Melee SMN is kind of and intelligence test. Nowhere does it say that you can do this, but if you stop and think about it, you'll realize that it is in fact like a match made in heaven. The fact that SMN is the only mage that can cast a spell once, and then live by that spell the rest of the fight, makes it stand out among other mages. Even BLU or DRK have a hard time to match the uninterrupted melee whacking of a SMN.

Then comes the usual problems. Lack of gear will make you hit slower than other jobs, even if they stop to cast occasionally. Lack of WSes with damage on will make you fall far behind in damage output. These two generally are summed up as "Feeding the mob TP". Since your really effective melee job, is really ineffective at damage.

There are some things to remember. Like if you have 4 melee, the monster has 100 TP when they are all at 25 TP... which is almost instantly. Sneak in and melee on SMN, and you have 5 melee, and the monster will have 100 TP the moment your melee has 20 TP. Which in time seen, is such a small difference that it will hardly ever cause more TP spam than it already was.

Other things to remember is that unless you are a taru, it isn't very likely that you'll die in melee range. I've really only died to things like NIN 2 hour move, and those things can hit you if you are incorrectly positioned in the backline as well. That said, I've obviously not meleed things like Orthus.

Most important of all is of course that it is the most fun way to play. Unless you dislike melee of course.