Security and stability are the big reasons. You'd also benefit from having the facebook and twitter buttons. You're also getting free help here, and at least personally like giving help to people who are current on their license (not like we can tell).
Some important changes you could quickly make to improve your search engine ranking: I'd change the title of your forum to "Lyme Disease Community Forum" and get rid of your image header and use H1 and H2 tags for your text. You also have two sets of title tags which isn't good. Google is having a hard time getting a good description for you in their SERP, so you might want to change your meta description.
http://www.google.com/search?client...+disease+forum&ie=UTF-8&oe=UTF-8
