I think it's a method of insurance for future development. If you start to get popular and get regular customers, do you want to share them with people who catch on to your website's popularity? Chances are, you will have a lot of SEO in place to keep yourself the top of your keyword heap, but the thing is, it would be a total disappointment if you built your site well and someone swooped in with clever SEO and ranked above you, taking your traffic, with same-name, different extension site.