How difficult is it to run dungeons as classes, not jobs?

[Vodomir]

Nope nope nope a thousand times no for Marauder/Warrior.

MRD/WAR: You gain your big tanking stance the moment you swap. You should never want to be a MRD again.

Unless you want to assume a DPS role. MRD gets access to more DPS cooldowns, so it might be worth the switch up until you get your relic and/or AF2 pieces, as those are WAR exclusive (relic and AF2 gear provide much more of a DPS boost than any cross-class skills possibly could; so MRD is not an option anymore, once you get access to that kind of gear)