I believe Internet marketers to be some of the most influential online website advocates. Why? Because we are avid promoters of other websites. Internet marketers will often suggest to their clients the use of other websites that add value to their own business. This added-value being links, but not just any links will work.
Incoming links are valuable to the receiving website both for generating more traffic and also for improving the position of a website in Google's search results. You may contribute an article, press release, image, video, or even comments to another website within your niche. However, don't expect the content holding website to always reward you for your contribution with a link to your website. They may just end up slapping a no-follow attribute within the code. Of course this makes no difference for the website user. For the link-builder however, this makes their contribution almost a pointless one. Adding "no-follow" to the linking code on a site, tells the search engines not to include any value to that link's website. At this point the resulting value may just be the human traffic that link delivers.
Since Internet marketers are more Internet savvy, as search engine optimizer they are likely to recommend sites that will reward their clients for their valuable content with a link that does not use no-follow. This means that without any reinforcement from Internet marketers it's likely that websites that use no-follow will not get the same amount of attention as the more generous sites.
If you own a website that features articles or other contributed content, consider what you may be giving up by using no-follow on the links pointing to external sites. Using no-follow may help sculpt your own internal page value but you may also be loosing some very valuable content contributors and potentially the real value of your site for your visitors.