It's really hard for you to understand that you can have tank classes with slight niche advantages while still remaining equally effective in general play. As an example, Warriors in WoW have a disadvantage in AoE tanking because they only have a handful of abilities with cooldown and resource restrictions. Bear Druids have a much easier time tanking groups of mobs because they have stuff like Swipe, which have low cooldowns, relatively low rage cost and can thus be spammed. Both are good single target tanks, but bears have a slight edge over warriors because of Swipe + Mangle + Demoralizing Roar + Provoke vs Cleave + Thunderclap + Shockwave.
That's a possible dynamic between GLD and MRD. Both could tank single targets at the same level of profficiency, while MRD could slightly have an edge on multiple mobs while GLD has the advantages that come with equipping a shield.
Not if having the MRD hold the targets in place is less risky than tossing a GLD in there. That would be like making a warrior to tank heroic shattered halls when the safer alternative is tossing in a bear or paladin tank. Again, this is all dependent on changes made to MRD to make this sort of thing possible.If you can't sleep the adds, even more reason for your best tank to get your job and the more dependable AoE'ers to get your job too. Marauder would be an undependable liability in such a fight.
So again, MRD needs changes. You're making this much evident. I'm proposing opening the option to be a tank on equal footing to a GLD with its own twist here and there. We should be doing everything we can to not repeat the crap of "one tank, one healer, 18 DPS" that FFXI suffered for so long.