Ok, for starters. All of you people, especially you RDMs who insult those RDM who do melee, please reconsider your stance after reading this. Melee RDM take a lot more work that people might be able to, or willing to put into, but for those of us who worked hard at learning to proper way, ease up!
To make this simple, no, a RDM should not melee, just because they can use a sword.
To be able to melee as a RDM you have to be able to play the mage side or a RDM to begin with. That means that you can keep your party refreshed, you know when to dispel the mob, you use other enfeebling/enhnacing spells appropriately, as well as Magic Bursting. RDM should expect to assist in healing, hopefully allowing other jobs to focus on it. Food and equipment is also an important factor, RDM aren't the top in any area they're effective in, so if you expect to main heal plan on food that is INT and MND+ or atleast MND plus, and if you expect to use Black Magics then focus on INT+ (sweet rice is a decent balance for a RDM). Finnally, learn to time your spells. You should have a general idea of when your spells are ready, and when they're going to need to be reactivated and be able to watch for that.
I will also say that RDM main healing is a bad choice if they have to keep more than 2-3 people (himself included), unless you don't mind getting refreshed after the RDM has to stop trying to recover. I typically won't do it, unless it's no more than 3 people to refresh, or there is another RDM or even a BRD. I will, however take it up as best I can when the situation demands it.
That being said, the next thing you need to know is how to guage your party. You need to know all the jobs in the party, what they SHOULD be able to do, and plan on a reasonable level, just cause you played with a WHM who could heal so well you never had to, that every WHM can do that. Sometimes that's simply a well set-up party and didn't require as much of the WHM.
The way I guage a party starts with the tank. NINs (be it NIN/WAR or WAR/NIN) tend to be easier (assuming they can keep shadows up with 1-2 miscasts at most every so often Utsusemi's). PLDs require a somewhat stronger set up, but it's certainly not hard to melee as a RDM with a PLD.
next, and the most important factor. What other mages are there. if both a WHM and BLM are in the pt, you're well set. Either of those job replaced while can still make a good party, RDM should take more caution when deciding to melee.
Finnaly, the DD. Typically this isn't much of a factor, cause RDM won't change much on the scale for this. either DD is weak, strong or just right. if it's too strong or too weak you look at needing a higher healing rate in the party, which means the RDM might be called on. Many DRKs for example are good at finding ways to steal , and when they get hit, they get hit.
This is a lot to consider, and most of the time many RDM can't guage their parties correctly, thus giving a bad name to melee RDM. When I guage a pt, sometimes I test it out. Please tell your party or atleast to party leader that you intend to melee, and if in the first couple battles it doesn't seam to be working out, then stop. step back and be a mage, that's more than 2/3's of your capabilities you might just have to do it (especially if you sub ANY mage based job. I find BLM to be best for melee RDM during XP parties. DRK might be possible, but you limit your spell casting when you do, so if you need it, it's not there, and the melee portion prolly won't make up for it, unless the other mages in your pt are awsome at what they do).
Next if oyu melee, expect to pay. you need the best melee food for your level. for me, I've been using sole sushi since level 59 if/when I melee in a pt. to do less would be a waste of time. With that and an En-spell I average 35+ damage, unless the mobs have awsome def. That includes melee gead (att+, STR+ DEX+ acc.+ etc.). Now, I have three full sets of gear practically(MP gear for start/convert, Magic gear for casting, and melee gear). the only thing I don't change is my pants XD.
Obviously I went the extra mile to melee, and do so responcibly. Those of you who insist on being rude or even insulting in a general fashion to RDM who melee, reconsider and realize that 'yes, a RDM can melee, but they better know what the hell they're doing'. Anyone with any questions can feel free to P.M. me, or post here. I'll try to get back to this post when I can.