In that thread, he the thread creator quotes a statement by Matt Cutts; top Google engineer as saying;
Google's view on this...Selling links muddies the quality of the web and makes it harder for many search engines (not just Google) to return relevant results.
This is a touch stance, does Google feel that all paid links should be discounted? Does Google feel that some paid links are not worthy link weight? We all know that AdSense links do not factor in to the link popularity equation at Google. We also know that Google hates link spam (comment spam, guestbook spam, trackback spam, link farms, link exchanges, etc.). But buying relevant text ads on high trafficked sites, is that worth any less?
Many of this debate goes back to some of the discussion at the first indexing summit at SES. Where Tim Mayer discusses a proposal for some sort of manually coded Block Level Page Analysis. It would be similar to the nofollow attribute or even the section targeting by AdSense. Bottom line, search engines want to understand where is the meaty content and meaty links versus the navigation, footer links and advertisements links.
The search engines are not looking to manually address this issue. It is too wide spread and we all know (using that phrase a lot here) that Google loves to solve these issues algorithmically.