Presumably, anyone capable of manipulating aether is capable of Creation magics, but most people don't know how to do it. The amount of aether the creator pours into it seems to determine how long the creation can last on its own without consuming external aether; primals need a constant supply of crystals and faith (which, itself, may just be another way of focusing aether into the target), while Emet-Selch's recreation of Amaurot continues to endure after his death (not to mention releasing Archeotania to roam the Tempest).

I kinda wonder how these revelations play into Fray and Myste from the dark knight job quests. Especially with Myste, who would seem to be a summon who can create more summons, like a primal creating adds to fight for them.