For what it's worth, Mrd can do some beastly damage at 50 with the right cross class skills.
Internal Release from Pug.
Blood for Blood from Lnc.
Raging Strikes from Arc.
All combo'd with Berserk and the 20% more damage from Maim and -10% SR from Storm's Eye means you can put out a crapton of hurt over the course of ~20s, or else stagger them across a fight for a larger sustained gain in damage. I mean, Maim + RS alone puts you at +40% damage. That's pretty nuts.
But if you're going to be tanking, you want to be a warrior.
that's all well and awesome. add in some DD gear and materia, and you are the DD MRD. If you stretch it enough, and level enough other classes to get those skills.... I guess MRD can DD. But still, it's impossible to get in any kind of dungeon or fight AS a DD MRD since you'll be pigeonholed into tank status by duty finder.
Likewise, you could cross class a bunch of defensive abilities as a Pugilist and sort of tank. Going back to ff11, I've had some epic DDs even healers makeshift tank when the main tank dies or simply have no tanks available. Especially the monks.
Got me thinking, 8 man event where half are job and half are class. Just using MRD and WAR as an example. Since 2 tanks are required for the fight, one could go WAR traditionally, the other DD MRD. That way, depending on how well the WAR is doing, the DD MRD could be a backup tank if the WAR screws up, or provide decent DD if he doesn't.
It sucks that dungeons and instances REQUIRE instead of suggesting party setups. It might solve the DD duty finder problem if they let some DDs class up, do less damage but be able to provide makeshift support.
Make a little checkboxes in duty finder that says something like "i want the recommended party setup" and "I don't care if my setup isn't perfect EFF IT~! I'll take my chances." It would make a lot of epic fails, but maybe a few rewarding unconventional wins for people determined to beat the odds.
Stuff like this would let players make the decision to include a MRD who is a slightly gimped DD, but still enough tank to provide "insurance" when **** hits the fan.
However, ideas like mine are potentially dangerous, as it might spawn a slew of generic, "do all" players that can do everything, but not that good.