Robert_Charlton - 2:22 am on Oct 4, 2011 (gmt 0)

there's a kind of decaying negative factor that spreads through the sites internal linking

I'm speculating that another way to look at this is that low-quality pages anywhere on a site statistically reduce the likelihood of user-satisfaction within the site. Similarly, inappropriate navigation choices statistically reduce the likelihood of user satisfaction.

Chances are that you get more positive points for user satisfaction within your site, but you at least avoid negative points if you don't send a user back to Google to run the search again.