It really depends on the site. Links from most sites are probably worthless at the the third level. However, if a sites homepage has a really high PR and the site architecture distributes the PR efficiently then a link might be valuable at 7 levels deep.
Here is another twist in the scenario. If I have a page that is ten levels deep and has 1000 one way links pointing at it, would you want a link on that page?
So my short answer to your question is, "It depends".
-Jim
P.S. I would gladly take a link 7 levels deep from
Adobe,
Google,
MIT, or
Nasa.gov