a friend and I were about to attempt this.. though what would we get out of it besides an insane amount of CR?
Outside the cR, if you beat campaign on Legendary... it's one of the requirements to unlock one of the armor permutations on Waypoint.
To me, that would be too time consuming and frustrating, especially considering how powerful the high ranking elites are as it is without all the skulls turned on in legendary.
Yeah... I'm getting my arse handed to me by the squads of elites on Legendary. All you can really do is sit back, try to find ammo, and camp.
Not really, In my experience 1 Mission of campaign=Same experience of 1 game of MM or FF. I'm not sure how the score effects your credits if it does at all but the skulls multiply your score not your credits. Test it out if all else fails you still can say you done it.