Link building is a very crucial aspect of search engine optimization. Search engines consider the number of incoming links to a website as an indication of their popularity and give them priority rankings. Many beginners fail to realize that it is links from authoritative and quality websites that are important and they mistakenly link to low quality websites for higher rankings. This tactic can cause the credibility of the website to go down with search engines and in some cases, the website may get banned.
I aggree with this. Many directories and link building schemes, require reciprocal links, this is detrimental to your site but many fail to realize it
That is why I need to moderate comments for my dofollow blogs, so that I do not allow links to bad neighborhood. I have filtered out many comments to those gambling, porno and other questionable websites.
spammers generally capable to find you blog place their bad neighborhood links at your blog that can harm your ranking