One example of how a tapered spar can help, could be seen in the testing of one of my UL kites. I originally framed the kite in non-tapered spars, except for the spreaders. The performance was far below my expectations. The frame was far too flexible, distorting noticably, and providing very little feedback. The wing tips shuddered noticably in any type of wind.

I framed it in tapered spars which, actually, showed more deflection in my tests. The weights however, were very close. Test flights revealed that all of the frame distortion was gone and the performance was much more in line with expectations. The non-tapered spar, while stiffer overall, allowed the entire leading edge to flex, causing the frame distortion. The tapered spar remained stiffer in the larger end, giving better support between the leading edge fittings. The thinner ends allowed for enough flex to retain durability. This could be especially important if yo-yos and "wrapped" flight are planned.

Another aspect of spar design that is coming to light recently is rebound or the spars ability to quickly return to shape after deflecting. A popular, high perfomance spar is said to excel at this. While remaining flexible and as such durable, it is said to return to shape extremely quickly. I have no way to prove or disprove this but, there appears to be validity to the claim. It does come at a cost as different materials and proccesses are needed to manufacture these spars.