Actually, those necks don't taper -- it's an optical delusion.

The truth is that you want strength at the heel of the neck, you want slimness where your fingers need to go the most, and you don't want a baseball bat for your neck, since you have to hold and play the damn thing. I suspect you could make a graphite neck that was slim all the way up, but then your picking hand would have troubles -- the wider spacing at the bridge would be missing, making you learn totally new string-distances from the traditional spacing.

Bob