Too put it as simply as possible, too stiff and your arrow will veer (usually to the left) and you will have problems tuning for that being it will not be consistent. You want your arrow to absorb and store the energy from your bow. Too light of spine and your arrow will whip erratically and will be pretty much impossible to tune. You need to find that "sweet spot" in the middle of absorption/storage, and control. Also, too light of spine you will run the serious risk of the arrow breaking under launch which you can ask MaTT (a member on this forum) is not a fun thing to experience. A piece of carbon arrow through your hand is not something to grin about.