A "Shatter Combo" is something Frost Mages can do to change up their casting against a Frozen target. By launching an Ice Lance immediately after a Frostbolt, so that both are in the air before the Freeze effect is broken, they both gain the benefit of the target being Frozen. This is an interesting casting mechanic, something that the Mage needs to react to in order to take full advantage of it.
Unfortunately, Fingers of Frost will not allow for a Shatter Combo, because it is a single-charge self-buff. It will be consumed as soon as you cast that first Frostbolt. And there's no reason to cast an Ice Lance or a Deep Freeze instead of that Frostbolt, because by the time you receive the buff, by the nature of chain casting, you'll already be partway through your next Frostbolt cast and it will hurt your DPS, rather than help it, to interrupt that cast and use another spell instead.
So, unfortunately, all Fingers of Frost gets us is a passive +5% crit on our Frostbolts -- if we've already spent the five points on Shatter. It doesn't change up our casting rotation at all, we will still be doing nothing but spamming Frostbolts.
The previous implementation, Winter's Grasp, did allow for Shatter Combos, but it was admittedly overpowered. Making it a self-buff was a good fix, but adding a single-charge limit prevents it from achieving your stated goal of permitting Shatter Combos. The best solution is probably to remove the charge limit and instead make it a very short buff, in the area of 3 seconds.
We'll be addressing this by allowing it to work on your next 2 or so spells, instead of just one.