Dave, the manufacturers don't "recommend 5 grains per inch". However, most will void your warranty if you shoot less then that. I'm quite sure that all manufacturers hope you shoot a heavier arrow. Heavier means less vibration, less noise, longer life for the limb, riser, accessories and string. Overall the shooting experience is more pleasant with heavier arrows and it's easier on the bow. Heavier arrows also transfer more of the bow's potential energy to the arrow. Hunters like this.

The only benefit I can think of, for light arrows, is increased speed. Increase speed can be a small benefit with 3D and some hunting situations, if you can retain the same accuracy level, and that's a big "IF".