True. Classes shouldn't be good at everything. I was thinking in broad terms of PT/solo situations e.g. How a MRDs multi-mob situation doesn't crop up very much when solo'ing, fighting the majority of NMs (single targets) and bosses like Orge (again, single target).

I do love how MRD can cause havoc across an area with moves like Storm's Path, but most high rank PTs currently require more single target DDs and MRD gets over looked. It could be argued that this is helpful in places like Dzemael where there are mobs all round, yet most PTs will skip the mobs to run to Ogre to kill him in time to get to Batrall, leaving MRDs multi-mob attacks kind of unnecessary.
MRD can still be a tank in those single target situations as you said but either due to GLA actually being better or the mindset of it being better (which I may be in also until I know it for a fact via witnessing both classes first hand or via video), people will still choose GLA because it outshines MRD.