I'm not 100% certain, but I think that the MediaPartners bot determines the context and keywords of your site for AdSense ad-serving purposes, whereas Googlebot gets data to be used for Google's search engine.
AdSense ads show up in the free version of the Opera browser - so if someone uses that to look at your site, the Mediapartners bot will take a look. Edit: Actually I just tested this and they don't appear anymore (perhaps Opera/Google have cancelled whatever agreement they had).