It will be a great tool for webmaster to block unwanted links spam links to their site. I don't know why is not still launching the tool in webmasters. It will be win win situation when the tool become live.

1. People could be spam links to give them a boost in rankings and then ask for them not be counted/trusted in a weeks time. Then simply repeat the process to keep ranking of spam links which you are telling google not to trust because you don't want them to harm your rankings (not sure how google could combat that really)

2. You could harm competitors (especially blogs) by gaining links on their sites and then reporting them to google as untrusted or spam links. Unless google is going to check each link that's reported this could be abused in order to damage competitors sites by making google think they are spam or poor quality links coming from their site.