It should not even be called a summoner class if all you can summon is one creature. The other trees can all summon one creature, so summoner is nothing special in that regard. Sure, you can buff up your summoned creature, but again that by itself is not a good reason to call it a summoner class. Also, for a class called summoning, there seems to be a lack of different types of creatures you can summon(just variations of incarnate based on what you summon it on). I can understand other hybrids only limited to one summoned creature, that is not their forte. However, the summoner class should increase the number of summons as you level it up so you can summon multiple creatures and not be bound to one.
On a side note, I also find the totems to be near useless. I do not even summon it anymore. Would rather replace it all together and have another summoned creature on the field.