It's also useful knowledge for spammers. If they have a list of people who are interested in topic X, then they can craft an e-mail with a subject and body that is relevant enough to topic X that the people will be likely to click on a link in the message. The reason that spammers do not already do this is because it is too much trouble to track the topics associated with e-mail addresses and because the number of e-mail addresses easily found for a topic is too small to justify the expense of writing a custom e-mail. With your proposed service, it sounds like it would be easy to both track topic associations and retrieve large numbers of addresses.
Really, it sounds like you're trying to solve two different problems: (1) given an e-mail address, find relevant topics and (2) given a topic, find relevant e-mail addresses. A solution to either of these problems has the potential to enable spammers, if it is standardized to the point where address/topic extraction can be automated. In this regard, it's hard to beat the current system of listing your interests on your web page (which at least solves problem 1).
Really, it sounds like you're trying to solve two different problems: (1) given an e-mail address, find relevant topics and (2) given a topic, find relevant e-mail addresses. A solution to either of these problems has the potential to enable spammers, if it is standardized to the point where address/topic extraction can be automated. In this regard, it's hard to beat the current system of listing your interests on your web page (which at least solves problem 1).