In a raiding situation, you take FoF and shatter (but not frostbite) and you just spam frostbolt.
You can't shatter combo FoF like you do with frozen targets, because its on a charge system. If you have one charge left, and you try to frostbolt and ice lance at the same time, only one will benefit. Which should be the frostbolt, but I wouldn't be surprised if latency made the ice lance benefit but not the frostbolt.
Ice lance doesn't do enough damage relative to frostbolt to use unless you are moving or for whatever reason are unable to use frostbolt.
Also consider you don't get FoF or Brain Freeze procs (once they make the change to chill effects proccing it, not sure if they have yet) with ice lance. Even if at certain gear levels frostbolt + icelance would be better than 2x frostbolt on FoF procs on paper, the inability for ice lance to proc these effects makes up the difference.