It has been discussed at length these metrics are becoming more and more part of the site rankings. If you look at adwords/adsense part of the bidding cost placement in adwords/or cpc in adsense is based on the sites metrics. Google indeed places a weight on this in the formation of the score your site is given.
"So the PR you see publicly is different from the number our algorithm actually uses for ranking. Why bother with a number thatís at best three steps removed from your actual goal, when you could instead directly measure what you want to achieve? Finding metrics that are directly related to your business goals allows you to spend your time furthering those goals."
This is speaking directly about site metrics. Furthering your goals is all about getting visitors, engaging, and converting whatever it maybe.