Good question, oh questions
1st you got blocked because you are spamming yahoo with links.
2nd it does not matter if a link is Dofollow or Nofollow. If a link is DoFollow then lucky you, that's really great news and you can increase the PR of the page you are linking to
But Nofollow links are also good to generate traffic not backlinks. For instance Yahoo answers can not get you backlinks but it can really get you tons of targeted traffic. This is why you also need Nofollow links, they don't count in search engines but they are good for quality traffic.
So don't just get any Nofollow link, make sure it will give you targeted leads