I've always though the Leonal would be an excellent summon, due to it's Heal, Lay on Hands, and Cure Critical Spell like abilities but I over looked something very important. They get Polymorph as an at will spell like ability at a 10th level caster. Think about that for a second. Polymorph at will, without the self only restriction other summons have on some of theirs. With my Binder Jacob that means I could be changed into any outsider with 10 hd or less due to being one myself already. That's insane and I can't even think of which form would be best for this.
This also means that they can keep healing every round until the summon is over, minimum of 18 rounds. Heal, Lay on Hands, cure critical x 3, polymorph (for target x hd hp back, a nights rest worth)
of course this assumes the players are allowed access to polymorph as a spell. at 18th level, 14th in my character's case, this isnt really too broken as is.