I think it's good information to know how long a person has been on a page rather than just page "hits." That would limit the effectiveness of paid traffic to increase a website's standings. But if Nielsen does it like they do TV ratings, where they have a small number of people with equipment that monitors their activity, I don't think they'll get enough samples. That's what limits the useful data from Alexa. If you don't use their plug-in, you don't count in their stats. So lots of people aren't getting counted.