I can see a few reasons. The biggest one would be that if you run two AdSense blocks, taking out the second (lower paying) one and replacing it with a YPN one may yield better returns.
Or if you prefer YPN to AdSense, you can still include the AdSense Link Units (which I don't believe YPN offers yet).
But, as noted, YPN doesn't allow AdSense on its pages. That's surprising given how Yahoo!'s latest quarter saw a dip in year-over-year affiliate site revenue. Ouch.