To put it simply, just because Sedna CAN heal, doesn't mean you should EXPECT her to heal you.
Too many times have I played and my allies have been taken out, then, no doubt out of frustration, a message appears "Sedna, where were you!", "Sedna, why didn't you heal!", "Sedna, WTF!".
To these people, if you have not communicated some sort of plan, please don't expect others to know what you're thinking and follow you blindly. Furthermore, there are many other factors to consider, such as Sedna's current Mana, Health, and immediate goals.
Having a support character as an ally can be helpful. You can be healed, you can be saved, etc. Just don't go expecting Sedna to stop what she's doing and focus on you.
k?