do you build a site to serve the search engines or visitors?
....and you have been on this SEO forum for 10 years! Why do you think that is? To find out what the search engines want of course!
Lets be honest, anybody reading this forum wants to find out how to improve rankings in Google. Over the years, it has been clear that the better you make the experience for the user, the more likely Google will rank you higher. Thats the new SEO, which has developed as google has become cleverer. However, Google owes us nothing and we are all subject to bugs in their algo or just plain google commercial decisions. Thats why we come here and discuss ways of maximising the chance that google will put us number 1. JesterMagic is quite right when he says we don't know what google considers a bad site, so you have to consider whether or not you let the spider through the link or not. Google loves links, likes sites with links and wants to follow them, but you increase the risk of being slapped for a bad link. You have to be cautious and to ignore this is crazy in my opinion.
if you believe you have to nofollow a link to a site -- therefore not vouching for it -- why bother linking to it in the first place?
Many reasons. I may want to make my page a good point of reference for a researcher. I may also want to credit a quote. AND, Heaven forbid, I may be doing this to make the site more sticky to impress the search engine! Bounce rate back to google may be a concern. However, the authority site in my opinion may have a very dodgy seo firm working on it. Therefore I link because of the quality of the text, and avoid the risk of the seo.
Google sometimes sends me marginally off theme traffic, which I may want to try and hold onto for analytics purposes. Providing links and helpful information may help improve my bounce rate for this traffic, rather than having a 1 second click back to google and the possibility that this effects my overall bounce rate. There are loads of subtle reasons why you may want a link to another site, and you feel that the link could help your visitor but be a potential risk as far as the spider is concerned.
Also, like any business, its competitive out there. Here's an extreme example but how nofollow is important and potentially helpful.....
Example: You sell Green widgets. Visitor from google comes in for Blue widgets. You send visitor to really rubbish, dodgy, expensive site for Blue Widgets. Visitor comes back and buys Green widget. You obviously put nofollow!
If you want to be helpful to a visitor but not pass pr onto a competitor, then why not put nofollow? If you feel a link is good for a user, but unsure about the seo of the site you are linking to, then do you avoid the link? Thats not good for visitors. The answer is use nofollow.
setzer -Most SEO is now visitor experience based. Nofollow is a very useful tool to use increase the visitor experience but not risk a penalty. If you can improve the visitor experience, you 'make ratings'.