To put it in simple terms--When you link to a web page, you are giving it a score of 1 in terms of search engine score.But when you use the "no follow" tag, the score becomes zero. So spammers wont get the benefit of increasing thier search engine ranking. Since, the web site publisher gets the choice to give a value of 'zero' or 'one', it is but natural for him to have the ability to rank his link as well.
By ranking a link, i mean to say that you canprovide a numerical value specifying how good or bad a particular web page is. Yes even bad pages should be capable of being ranked. As of now no search engines honour the ranking of links. But inthe course of time it should become a default feature not only with search enginesbut also web publishing tools. Since we are addingit to the "rel" tag, the scoring should be doenas follows:
rel="score=2" for a positive score of 2
rel="score=-3"for a negative score of 3 etc...
Arguments as to the effectiveness of this are plenty. The rank tag will become more useful as we enter the era of personalised search. Your blog, mail,social network...etc..will be indexed to provide you with personalised search. When this happens, the use of scores will not affect the overall ranking of a website,just the ranking with respect to a particular person.
Other attributes may be added as time progresses. The process of ranking links can be automated to provide ranks to pages based on the domain name of the page. The various bookmarking sites like delicious are involved in collecting the opinion of different people on a particular web page, the use of scoring is hidden in these sites. Although different people have different scores on their tagged pages, it is hidden and is calculated based on various criteria. The idea of scoring web pages, is open in google coop. In the customized search engines, you can directly provide scores to sites. Thus, it is useful to score web pages directly in the links.