Search is about leveraging users' interactions. Every time you submit a query and select a few links You provide valuable information to the search engine that will be later used for ranking. Normally this information are intrinsically provided. However, this is not the only possible interaction model.
For instance, imagine that you are a top IT executive with the secret passion of snowboarding. Why don't allow you to answer to search queries related to this topic?
Another example, imagine that you are a banker working in finance with the secret passion of interior design. You'd love to share your expertise with other people for free but with current search model this is not easy.
Today, everyone has a linkedin profile. So why do we extend the search model so that I can register myself for a specific search query and offer my expertise on that topic for future users who are submitting the same query?
I think that the problem of spam is completely avoided if we adopt Linkedin profiles for registering users.
Also, I think that users would love the serendipity of being in touch with other people sharing same interests.
BTW, did I say that I love acquarium, ham radio and late 80s music?