1) That Google and other search engines don't devalue obviously paid links. They corrupt the search rankings in many cases. And it can't be difficult for the engines to identify a large portion of the paid links.
2) That people at Webmasterworld defend paid links and compare them to advertising. There's a reason newspapers force advertisements to be printed in different typeface than their news stories. It's so readers can tell them apart. Paid links distort the search engine results.
And how do the above comments not pertain to link exchanging schemes?
Why would they (SE's) devalue links/advertising that are paid for and not links that are exchanged? To me purchasing advertising/links are a whole lot more natural than exchanging them... when was the last time you saw advertising in any other medium that competitors advertised back and forth with each other? I can see it now.. Ford Motor CO. having GM logo's in all their brochures and visa-versa. People that believe that exchanging links is fine, but buying them are wrong, must just be cheap or something...