I don't see why not! If a Druid can morph into a lil' cat and tear butt away from me, why shouldn't I be able to get up on my war steed (seeing as it IS a castable spell) and run him down like the nublet he is? Sure, I can see if some form of damage interrupts the casting, but doesn't that seem fair and logical to be able to do that?
PS. I want to see lances in the game. Massive crits with a cavalry charge of war mounts = beauty on a Prnt Scrn