the introduction of no-follow rules

Has been the debate over paid links happen for a while, 'and it seems that there is no stopping in the current debate. First of all what has been paid links?
Contact paid when paying for a link on a Web site. This in itself is not a problem and the search engines do not have a problem with this, because this is advertising on the Internet, advertising and building. Problems arise when an advertiser pays only for the connection in order to increase PR there.
This is a problem for search engines such as this in an attempt to artificially boost the power of your web site. Google provided the first move of defence against this tactic with the introduction of no-follow rules. This means that if you put a link on the site and pay on this link should be no-follow attribute on links that are placed to inform the search engines that they paid links are used only to build traffic. What is not instructed no-follow search engines do not follow this link to the recipient will not receive power. And can also be used no follow for other purposes within your web site to distribute better Page Rank.
Yahoo did not make a concrete decision on this issue, because, at the time of writing this article is to ignore the rules of no-follow  However, stated that she believes that paid links are of low quality to the user, and I agree with them. These types of links that do not sound right to the site, why are there only for profit.