Summoner is easier to rationalise than say White Mage. All it requires is the following: to have a soul crystal, to have defeated a primal, and to then defeat/subdue the essence of that primal. As was discussed in another thread, beating a primal doesn't always require the Echo, because not every primal tempers/drowns etc. And the soul crystals, although rare, are numerous, as shown in the summoner quest.
Aduu Avagnar, The Wanderer: Wiki