Yeah... I just think that if I can relocate Garrisons freely than there are few or no consequences to my decision as to where to put them. I'm fine if returning them to a pool has a industry cost... thus costing time and opportunity. Prevents you from shuffling that legion across the empire too easily.
Also I think of legions as spear-tip forces that can descend on one point of the planet, whereas Garrisons, being the same amount of manpower, but having to be capable of defending anywhere on the planet, have divested their forces and spent resources on fortifications and automated defenses instead of the cutting edge offensive weaponry available to a Legion proper.
But its all up for opinion.