This question is tricky as well as simple to answer. I would like to share a Moz post written by Jason Lancaster - http://moz.com/ugc/the-nofollow-tag-...a-drunken-boss
In the above thread, I liked one of the comments which I believe you might be interested in.
"I think that nofollow is meant to say Google that webmaster is not ready to share his page's Page Rank with that particular linked page. But Google can give some value to the linked page without taking it from linking page.
I really believe that Wikipedia don't lose anything with nofollow links, but linked page is getting a lot from this nofollow link."
So did you get your answer?