[dzdt]

I read the details of the advantage differently. Not that it gave a better killing blow, but that it increased reusability of the spear point.

It acts as a crumple zone, like we have in cars now to protect occupamts in a crash. For the spear points, the fluting "crumple zone" protected the important cutting edges from breakage. The shaft would break loose at the fluting, damaging a small amount at the butt of the stone but allowing the main body to be reused.

Without fluting, at a hard impact the breakage was more likely at the tip, ruining any potential to reuse the point.