Here's how I vision a spell binder specialty...
It would be a pet focused support dps class relying on AI driven summons.
It would have 4 primary summons each of which focused on demons and benign spirits, them being:
1)Spirit of faith- A healer summon that would have its' own aura of healing along with minor AoE heals and the ability to cast ranged auto attacks but be fragile with little HP points.
2)Spirit of Valor- A buff summon that would reduce damage to allies in its' vicinity and cast melee ranged spells with large hit points but low damage would also cast a weak barrier on everything around it.
1)Sloth demon- A demon that deals high melee damage with medium hit points and debuffs everything with an aura
2)Fear demon- A demons that deals high ranged AoE damage and medium melee damage with low hit points but casts its' own barrier that has a lot of hit points.
All of them deal spirit damage and the passives would revolve around empowering the demons and benefiting the caster when they have the demon/spirit out.
This is all just out of my imaginative head though...and chances are that this would make spellbinders "op".