There are a couple of scenarios where it makes sense to cast scorch.  First, in world pvp you can cast it and not give your position.  For example, say your behind a tree and casting a frostbolt, the animation will show them where it's coming from.  Scorch doesn't have that, so it's a good way to not give away your position.  Second, it's probably the best dps when you are frost locked by counter spell.