from Wordpress's terms of service:
Automattic has the right (though not the obligation) to, in Automattic’s sole discretion, (i) reclaim your username or website’s URL due to prolonged inactivity, (ii) refuse or remove any content that, in Automattic’s reasonable opinion, violates any Automattic policy or is in any way harmful or objectionable, or (iii) terminate or deny access to and use of WordPress.com to any individual or entity for any reason.
I've seen this clause enforced against people in entirely ridiculous ways, and also by mistake, which Wordpress has retracted after doing much damage to a blog and its traffic. If they don't like you, or if a computer glitches somewhere someday, they'll seize your blog and lock you out with no warning. It's sadly apt that the parent company's name is 'Automattic'.
I'd just make sure to have a very solid backup if I was using them, and a plan B url.