I recently ran into someone using that phrase in a discussion I was involved in. In about half a second this jumped into my brain:
Because the truth is there is no such thing as "balancing" faith with reason, at least not the kind of "faith" we are talking about in the context of religion which is where this phrase is inevitably utilized. Faith is UNreason. Faith is what is resorted to when you realize reason isn't going to support you believing in that thing you really want to believe in. But people don't want to look at it that way, so they instead try to make this silly argument that if they mix in enough reason over here then that it somehow mysteriously offsets the areas where they're not being reasonable over there.
Sorry folks, doesn't work that way.