I support this idea 100%!!! I HATE having to click all my spells to clear my deck because that usually takes a few minutes that I could spend doing something more interesting.
I'd probably never use it, but I could see others doing it. I generally just start top left and keep clicking first spot. There's always certain spells you're gonna put back in anyways.
As a player that mainly uses one deck, I am constantly altering my deck for different PVE mobs and bosses. Not to mention when I venture in the arena. An empty deck button would be highly useful for me.
Excellent idea markopolo, I hope it makes it's way into the game one day soon.