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Some Facts (possibly debatable)
So if they don't, why don't they? it would IMHO improve the quality of there results.
If they do what usability factors do they look at?
In practice it kind of worked...IMHO, i.e it always liked my sites;) It used to take months to update tough, which meant new quality sites weren't seen for a long time.
However, it was never popular enough to survive the SE wars and ended up in the SE graveyard. Try DirectHit.com today and you end up with Teoma (another engine I like), and its new owner. Teoma has a more traditional ranking algo which doesn't use the old DH technology.
One problem with "usability" is that it is very difficult for a mathematical algo to measure.
You could argue that Google/Inktomi and a couple of others measure "usability" to some extent in that other sites have to give their approval of quality by giving a link to them.
Most major engines now consider link popularity in their algos. Having said that, so many people are link farming, grovelling, cross linking, guestbook signing and performing numerous other activities to artifically gain link popularity that it is becoming a bit of a nonsense!
Relevance, usability and quality are often beauties in the eye of the beholder. Whatever method SE's choose to attempt to implement these factors there will always be more upset people than pleased ones!