I'm currently leveling Mage X 5 and I have to say that it's completely worth it in that case. As previously mentioned, even if the caster doesn't get to use his own frostbite, one of the other mages will!

If you're only running one mage, I would highly suggest still taking shatter, skip frostbite, and just pick up Fingers of Frost later in the tree instead. You'll get a very similar effect without worrying about your other characters breaking the root early.