Fire lacks mobility and has long casts. There's really no benefit to it. Of course, you'll do fine against the f2p's that you generally face.
instant Pyro shatters are worth it.
Either way,, I'd not be happy to stand around hoping for insta pyroblast procs, then trying to set up shatters while the proc is still up without a pet nova. Point being, the spec is clearly inferior to other mage specs.
Tell me why its not viable, scorch is not a viable counter nor is crit when it can get to 40% respectively.