The two biggest factors that make Crystal Fragments a viable and strong spammable skill that Magicka Sorcerer DPS can now use are:
1) The cast time for hard-casting Crystal Fragments (no proc) is ~.8 seconds which is less than 1 GCD (Global Cooldown) which is about 1 second.
2) Post-Stonethorn, Crystal Fragments can now proc itself.
In terms of numbers, it is quite comparable to other spammables such as Force Pulse and Elemental Weapon.
An example of barsetups with using Crystal Fragments as a spammable skill is shown below: