Because it's a stun and due to Shatter
(All specs now) you are almost guaranteed crits on spells cast on deep froze targets. As fire, you can inferno blast>Frost Nova>Deep Freeze>scorch>Pyro>Combustion>Inferno Blast and your target mob and surrounding mob or 2 are gonna be near dead. When you pyro into a deep freeze it's gonna be a crit almost every time (and can be everytime with good crit rating). Also Deep Freeze is only a 30s cooldown now so it's got great uptime.
Edited by Vladamyr, 30 October 2012 - 01:37 PM.