It seems like the shared cooldown between ES and FS, while perhaps serving to separate the true cooldown masters from everyone else, doesn't really serve any useful purpose, and is left over from earlier iterations of the game when perhaps it made sense.
I feel like Flame Shock should have no cooldown and VERY VERY tiny direct damage, just like Glyphed Riptide (or 'Water Shock', as it ought to be called), where as Earth Shock can retain it's short cooldown.
Perhaps someone can explain why the two spells sharing a cooldown is useful or important?