The Arcanist and Summoner are disconnected from each other and Summoner's extension from Arcanist seems forced. The Summoner harnesses Energy of the Primal's and reshape it into a biddable Avatar which translates to Egi in Allagan. There is no reason why Summoner's don't have Elemental spells since they can channel the Energy of the Primals. I'm pretty sure judging by the story Summoner will get the ability to reshape the Egi into the Primals just like White Mage will get Holyra and Black Mage will get Flare Star.