It's a good question, the answer is prretty simple. To some degree a subdomain protects us from duplicate content issues. The sub is just as strong as a primary domain and with a project such as this if people start spamming it's very easy for us to cut a sub without impacting the main site. Also with a sub you get the advantage of having the main search term at the start of the url which is good for search both in terms of users clicking through and SE weight. Are you going to take up Vegas?