When does it make sense to use NOFOLLOW on internal or external links?

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You do not gain any advantage by using the nofollow attribute for internal links.

I would not use nofollow on internal links
– Matt Cutts, Head of Google Webspam Team

 

When it comes to external links, the nofollow markup can be used to help prevent spam on interactive pages, where visitors are able to compose their own content. This way, visitors will not create spam in order to add a link to their own website into the content they create.

The Google webmaster guidelines also explicitly point out that paid links or advertisements on a website have to be marked with the nofollow attribute.

Search engine guidelines require machine-readable disclosure of paid links in the same way that consumers online and offline appreciate disclosure of paid relationships (for example, a full-page newspaper ad may be headed by the word “Advertisement”).Google Content Guidelines

 

If your website is getting (many) backlinks with the nofollow attribute, it will not have a negative impact on your site. Nofollow links are neither relevant for the PageRank calculation, nor are they seen as a negative signal.

Video explanation by Matt Cutts / Google on the subject

Further information on the subject:

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