About.Me Has 400,000 Beta Testers, Opens Doors To All

One reason is search engines. With a 301 redirect for example, you are telling them that your content has permanently moved from example.com to about.me/example. Your efforts to promote your page and acquire backlinks will only aid about.me not your own domain. If about.me were to shut down or you were to decide to build your own homepage, you'd be starting from zero SEO-wise. Furthermore, people may bookmark about.me/example (it's where they landed after all) or link directly to it bypassing your domain name.

There are also other considerations, mostly related to the user expectations and experience.

Your own TLD is a lot cooler than a directory.

Not really, other than preventing another "My Name" from ranking higher in search results for your name than you. It's a land grab.

In my case, I was able to get a shorter URL ronnieroller.com vs ronnie.me. Makes it easier to say over the phone, twitter, etc.

Make sure you set up the redirects properly. Having multiple URLs that point at the same content could penalize you on search engines for duplicate content (I believe).