Yes, you've noticed one of the aspects of frost mages in PVE. Frostbite + shatter = awesome. But keep at it and you'll start seeing bosses/mobs that are immune to roots/slows and when that happens, your DPS drops a significant amount and when the shat does hit the fin, you have almost 70% less burst damage to pull out on short notice. It doesn't seem like much but it adds up fast. Granted, once you overgear everything, it makes this decision much less important. :P