There has been a lot of talking about whether rel=”nofollow” is good or bad, useful or usless (check
Bitflux Blog :: Why nofollow is useless )
However I think that nofollow is quite useful for a different reason than preventing spam: preventing search engine results shown on “third party” websites mislead the search engines themselves.
Here is what I mean: Take for example g-metrics.com. Most of the pages of this site display results (actually the top-10 results) returned by google for specific queries. To do this, g-metrics.com makes use of Google Web APIs. So when google indexes one of these pages, it would increase the pagerank (or whatever ranking system a search engine is using) of a URL just because it had high enough pagerank to show up in the top 10 results…
However, using rel=”nofollow” can prevent this. (I have started using nofollow in g-metrics as soon as I read about it).
Similar cases could become a big problem for search engines, as similar APIs become more popular (Google, Technorati, del.icio.us, Amazon/Alexa, etc…) The way I see it, similar services may oblige or encourage developers to use nofollow when presenting “their” results.
I’ll admit that you’re scenario can cause problems with the PR system. But the problem with the nofollow solution is that it will cause more problems than it will solve.
PR is calculated by how many links go off your site. For instance, if I am PR5 and I link to 3 sites, my PR will add to their PR. If I have 10 sites each of the sites get less PR. If I have 100 links and but all but 1 have nofollow on them, what if that one site is a Pr0n site totally irrelavant, but nicely hiddend and funneling ALL my PR away?
This is already being used (not abused, because this is what it was made for), and it will totally breaking what the PR algorithm was good at.
I personally find nofollow useful in some situations. It is very useful for conserving bandwidth if you are linking to pages that are marked “noindex”.
While the use of “nofollow” won’t solve the worlds problems, there are some situations where it makes life a little easier.