Maybe I didn't pay enough attention, but I never understood the lore defense of the egis. Correct me if I'm wrong, but even if we did summon full primals:
A) Without the massive ammount of crystals that beastmen gather for summonings, they should not be as large/aether consuming (assuming of course that they eat aether to sustain themselves and that their body sizes should dictate how much they need). Like another topic said: roughly 25-30% the size and scale of the primal should suffice, 50% for limit breaks (if they ever became class/job specific).
And B) When a primal dissapears, all the aether they've absorbed returns to the land. So unless a summoner keeps his Egi out 24/7 there shouldn't be any adverse effects, right?