I think she is the least played hero when ranked chosen heroes. In all my games, I can only remember 2 fight vs her and she lost.
if she had 2 more def i would play with her.
Undoubtedly, the reason she is played less than say Fighter Mage/Sorceress is because she's a premium hero. The Summoner is exceptionally powerful which is why we had to reduce her Defense from 13 to 12 in the last balance update. The ability to summon an endless horde of creatures allows her to relentlessly advance on the enemy from the safety of a secluded area. When you don't have summon cards, you can produce up to 5 minions for only 1 card a piece. And, her 15 Valor ability is the equivalant of always having a Healing Touch whenever you need, making her one of the most difficult heroes to kill if used properly.
Raising her Defense by 2 would put her on par with Fighter Mage which would drastically imbalance her (especially when coupled with her unlimited healing ability). As an alternative consideration, what if she could use her summon skeleton ability more than once each turn? I think this would add a lot more fun to her gameplay, but this also raises balance concerns as well as inconsistencies in the "1 action per turn rule".
We are planning to add new "unlockable" powers to each of the characters that will allow players to swap out various abilities to customize their Heroes. Maybe one of these powers could be "Catalyst: Summoner may use Skeleton Warrior more than once per turn." and you could swap this out with Sacrifice or Summoning Mastery.