From my personal perspective I only purchase links from other sites when I think I'm going to get enough real traffic from them, focused traffic, so that the links are profitable for me. And yes I track that stuff and will change things if they're not. If there's any ranking boost to be had, that's all good. But that's not why I purchase advertising.
There is an interesting discussion out there about those "crap" links you see being sold on some fairly authoriative sites. The main question revolves around how these paid links are handled by the search engines. eg Whether they pass reputation or not and if they can actually end up hurting the site that is accepting payment for link placement in the long run.
Okay, that's the preface and a bit of context...
One of the best discussions on the topic I've seen is over on Tim O'reilly's blog where he's been struggling with the subject somewhat, as it can be a moral dilemma. It's a long read even though it's only a week old --and quite convoluted as blog comments can be-- but there are a few gems there that may make you sit back and go hmmm.
The most interesting blurb to me is the second comment from Matt Cutt's of Google, which is something I've always suspected but not seen confirmed in any way by a legitimate source before. To partially quote him, Matt said this:
Emphasis added by your's truly.
Honestly, such a stance makes absolute sense to me, given Google's background. I can see why they would attempt to come up with a way to devalue some of these paid ad links that are totally irrelevant. But it's interesting to see that they're actually doing it.
My question then becomes, at what point does paid advertising --that is out of control of course-- actually start to hurt the linking site's reputation in their own field? Or should they be able to get away with it, simply by the search engine devaluing selected outgoing links, but leaving everything else untouched? Is it an immediate thing, or should it be, ala the old Link Farm debate where the entire site gets hosed if even a fraction of the pages are participating in a Link Farm?
If one is looking at intent those questionable outgoing links may certainly tell a story, regardless of whether they're being paid for or not. I know that as a surfer I mentally ding sites that show this type of thing, even if the rest of the site is top quality. I wonder if the search engines shouldn't be doing so also?