The problem is this opens up the possibility of negative SEO. Just start making random links to your competitors' sites instead of your own and it will negatively affect their ranking as google have admitted recently[1]. This just puts us back to square one - at least it seems to be the case that no matter what Google do, spammers will find a way to break it.

[1] http://support.google.com/webmasters/bin/answer.py?hl=en&ans...