Right now, MRDs are still tank/dps hybrids. They're not the best when it comes to melee dps. Lancers have that gold medal right now. They're not bad at dps, but you have to understand that if we're talking about endgame, right now, the only endgame fight worth mentioning that melee dps can do is Ifrit, and even though THM are preferred there, you can take a few melee dps and not worry about it. That being said, as the game stands, you'd need to level an ARC, CON, or THM to find yourself in the Moogle fight, and most people have multiple classes leveled to 50 so they can participate in that fight. With new dungeons coming out very soon, it's possible that multi-class participation will be revived, which brings up the original question.
No one really knows exactly how jobs are going to fair in comparison to each other, but looking at the abilities that are being given to MRD, they're still trying to make it into a tank/dps hybrid. This isn't a bad thing, as you can take a lot more dmg than a LNC or PUG (which does a lot of dmg now too), while still dealing a decent amount of dmg. I guess this whole post has been a long ramble boiling down to the general idea that with jobs coming out, we can't put stock into how things will change, but as of now, MRD is built to do some pretty nice DPS, but is also built to be tanky. If you pick it as your main and things don't change, expect to fall behind lancers and pugs in dmg for now, but no one is going to ask you to tank endgame content. As far as smaller groups for leveling and such, you'll probably be asked to tank if there isn't a glad there, and even then, you'll be expected to off-tank.