I've gotten into some ugly arguments with people about magic in fiction about just that. Some people really do want/need to have everything in the fiction become predictable, measurable, and therefore conquerable with metagaming. I think much of the appeal of magic is that it's supposed to defy consistent measurement. Yes, there's a blurry boundary where "magic does the unexpected" becomes deus ex machina laziness on one end, but I still prefer to be near that side of the boundary.