I use them because of the mechanical (front derailer) setup on the Pedersen as it relates to gearing.

My front derailer cannot be lowered below the height for a 52t round big ring.
With BioPace rings, I am using a 50t big ring, which is the one ring I use for almost all of my time spent racing and road riding.

While BioPace HP rings escape my notice while I'm riding, I have noticed a slight improvement with them (in situations where I might stall out and have to unclip to save a fall). I just hang in there and pedal with some extra confidence that I can pull out of tight situation, as when re-mounting awkwardly in technical terrain.

In shorter mass-start events like cyclocross, a strong start is more important. Here, I appreciate the ability to shift earlier without feeling the spike of too much resistance at the pedals (and my knees). The earlier shifting gets me up to speed faster since it eliminates the sharp fall-off in pedal torque as the cadence gets too high. This is exaggerated by the lag in the actual ratio change from when each upshift is initiated, so each upshift gets me another half-bike-length ahead of the guys at each of my elbows. My STI-using competitors have it even worse, because their upshift isn't initiated until they have pressed and released their STI lever, while my thumbshifter moves the cable more immediately.