It's explicitly stated to be OK in their own FAQ:
> you can send any legal, non-spam email through Mandrill, too
and on http://mandrill.com/about/
> Use Mandrill to send automated one-to-one email like password resets and welcome messages, as well as marketing emails and customized newsletters.
It also doesn't appear to state that presently.
> Mandrill is an email infrastructure service designed to help applications or websites that need to send transactional email like password resets, order confirmations, and welcome messages.
> Any bulk email should be sent through MailChimp, rather than Mandrill.