According to Matt Cutts, the Adsense mediapartners bot is helping Googlebot with indexing. How? Well, this was confirmed by him in this week's Pubcon. In his statement he said that Adsense mediapartners bot is not only focusing on ads for Adsense but also indexing for the regular Google search database, in a bandwidth saving move. Hmm..quite an important thought to think upon.
The above statement is justified by Matt by saying that there lies no advantage in indexing pages by one bot or the other. By this he means that the frequent activity of serving various pages to each bot can be source of inducing problems in the search index. For more details visit Jensense.
The whole matter was confirmed when both Greg Boser and Matt Cutts saw that the pages were being served directly to the
Adsense, which eventually started showing up in the main search database. Refer SEW blog.