Welcome to the Q&A Forum

To add to the other responses, I can see reason why you might want to not use your keyword in a sub domain.  If you do, it means that everytime you give you web address out to someone you have to use it! Depending on your keyword, this could be a nightmare! I'm not sure it'll do your "brand" many benefits either. What happens if you go down this route and decide to change your target keyword, or your keyword is no longer relevant to you business (things change!). Do you build a new site hosted on the subdomain for each keyword? (Things could become silly if you take this too far!) A simple "www" or nothing has become an easy to remember convention.