What is the purpose of the rel=nofollow tags? To put it simple, they were created to prevent people from manipulating Google PR (PageRanks).
Nofollow tags were created especially for comments in blogs or forums where the site owner might have little control over the quality of the links posted by other people. They were certainly not created to be added to the contextual links within an article.
If you quote a website in your article and link to it to show your readers the source, it is unethical not to give it full credit just to save your link juice.
The nofollow tag is not really necessary when you moderate comments and are sure of the quality of the links posted.
Created to protect blogs against spam comments, the nofollow tag doesn’t discourage spammers. They still spam. So the main purpose of the tag failed.
Comments bring value to a blog entry: a witty statement might lead to days of debate. The text of a comment increases the content of a certain page, thus increasing the editorial value of that page. It is more than sensible to give credit to the commentator who was kind enough to boost up the debate and add contextual value to your blog. A link back is a vote of quality and a thank you.
More and more SEOs are against “nofollow” tag unless they are employed on user generated content. A very good entry related to this topic you can find on Graywolf’s blog.
To summarize: I always recommend it to my clients to create their web pages for the people. Nofollow is a tag created for the search engines. When you use nofollow for all the comments you receive you discriminate legitimate users as spammers.