You bring up a great point, shaners -- testing. I'm guilty of not testing all my creative, so I know there must be others out there.
For the most part, and if the affiliate program shows promise, then testing the creative should be moved up the priority list.
I should probably add that the purpose of a banner advert is to get the click through -- not to sell the product, that's the landing page/merchant's job.
So, testing it should be likened to testing a PPC advert. Which tweaks increase the click through rates and such.
~ Teli