I love Assize. It's a great, fun ability that feels awesome when you use it. That said, it has a touch of awkward design.
It does three things: AOE heal, AOE damage, and self MP recovery. Those are three awesome things. However, two of these three functions encourage you to use it whenever possible, and the third (heal) encourages you to sit on it until you need to use it. The two non-healing functions are strong enough that it's better to just use it on cooldown and GCD heal afterward than it is to sit on it for more than a few seconds. This functionally makes it unreliable as a healing ability.

Putting it on the two charge system would fix this almost completely. I can see balance concerns since it would allow a double Assize on an opener, but outside that it would function the same way, except you could sit on its timer for its healing side without losing out on its damage or effect on MP economy. Maybe disallow building the second charge outside combat?