It has benefit to your site because do-follow attribute is for Google only but no-follow is crawl by other search engine sucj as yahoo, ask etc.
No follow attribute in a backlink means visitors to that site can come to backlink site by clicking the link, but the search engine spiders can't. So it does not confer a SERP advantage to your site but can still bring Traffic. Do follow is the back link that brings human visitors and helps on SERP also. So, if you are linking for the purpose of SEO, better to go for do follow links otherwise no follow is not a bad option for getting classified traffic.
that's not right. Spiders can and do follow 'nofollow' links, even the googlebot. It's misnamed - what it actually means is that to the google algo... don't pass PR down the link