There has been a perception in the minds of SEO companies & webmasters about nofollow links.They think that nofollow links are quite useless and their websites will not get any benefit out of it.
No follow links are those links which doesn't provide outflow of PR juice from the original web page.For e.g. if a page has Google PR4 and there are many links to other sites/pages on that webpage,all those sites/pages will get PR juice from the highly ranked parent page.
However, by using nofollow tags,the website owners stop flow of link juice to other sites/pages.This is because the search engine spiders will not have the instructions to visit those linked pages/site and therefore they will be limited to one page only.
But many SEO companies forget,internet is not meant only for spiders/search engines as the very important role of users can't be ignored.A website should be built from the perspective of users primarily and not search engines.The links with nofollow tags will still get benefit as the users will not be stopped from directing to the linked web pages and thereby increasing the traffic for those pages too.To make it more simple,let's take an instance,a web page with high PR and having number of links to other websites/web pages but each link has nofollow tag which restricts spiders to crawl them,but the users will still be able to view those links and they can visit them too.An expert SEO company will always focus on building relevant back links irrespective of their follow tags.
After all, we build & promote websites for users and not only for search engines.So my strong advice would be to build as much back links as you can and don’t worry about nofollow links and this will be beneficial for your SEO program.