We currently have no Summoners in the Free Company, and so if in future they become a vital class for progression, we would perhaps find ourselves at a disadvantage.

Also, as it stands, there is no reason why a Summoner should not be included in any of the current content as their DPS output is more than adequate. Aside from that, pets bring their own unique advantages such as counting as a player during Turn 5, where they can be used to absorb damage from some of Twintania's abilities.

It is foreseeable that similar mechanics could be included in content in the future.