Look up the equivalent per click earnings you get from affiliate programs. Then compare that to how much the same merchant is prepared to pay per click at Google or Overture.
It is not uncommon to find merchants paying $1 a click at PPCSEs, yet affiliates find it hard to get more than a few cents per click from that merchant's affiliate program.
In such cases, AdSense makes more sense.
And it's one of the reasons why AdSense may spell bad news for all the ripoff aff progs out there.