I think this makes sense whether or not the taper is a straight line or slightly curved. I think the flaw comes in the practicality of it. I believe about all of the pyramid bows I see don't taper to a point but to a wider tip which means that the force on the limb is going to change. What we consider pyramid bows almost never act like a truly pyramid shaped limb and therefore are going to need some kind of taper to achieve that circular tiller. Considering that it isn't a truly pyramid shaped limb then that circular tiller is technically incorrect for the limb shape too for efficiency of mass distribution. Not sure how the tiller of an even thickness elongated trapezoid should be though. 
This was covered way back in the thread. If the triangle is drawn to a point the tiller will be circular, but since that leaves no room for nocks, you are correct that some taper somewhere is needed. I usually end up tapering the belly of the outer limb as needed.
BUT, it is possible to draw the side lines to the point but stop cutting along them where they converge to the desired tip width. The tips are parallel from that point. Then the whole limb bends but the tips bend a lot less, which is generally desirable to improve the string angle and return speed.
