I believe,"Dualcast" is the key here.Whenever caster move,its dps loss but Dualcast can bypass that and that's why RDM can sustain their high dps while handle the mechanics.They've got access to this "infinite swiftcast" after all.
For casters,high mobility should come with low dps output.its the price to pay but mobility will help sustain dps.
Too bad RDM get both mobility and high potency skills also various raid utility.

I'm not gonna say RDM should get nerf or BLM/SMN should get buffed.
Its not my call and I'm not taking sides.

And what If RDM don't have Dualcast though..Are things gonna be much different ?