Convert is actually a great tool to use while you're during your main burn and are running short on MP. It allows you to maximize your damage output while under cooldowns or to use different rotations effectively. Apocatastasis is a very underutilized tool but is useful in a few endgame scenarios. Manawall is great for when you take aggro or just feel like pulling and negating melee. it will also be extremely useful in pvp, as will the above mentioned skills.

Freeze is a light damage CC. it binds the targets in the circle and is supposed to leave a effect for 15s that when entered binds anyone who enters. Hope that answers you.