The component doesn't have support for merge tags, because I'm failing to identify the uses of merge tags in the joomla context.
What we put in your main content is normally something similar to merge tags. Out of a php variable was generated a text. This article here:
gives an example with first name, email -> but those for example are generated in joomla, there is no need for them.
In your case list:address, list:description? What kind of list? Mandrill is a transactional e-mail service - it sends 1:1 emails. I fail to identify the list in this example?
We don't offer unsubscribe as we can't make this 100% reliable. I can add a table with unsubscribed users, but I cannot garantee that all the e-mails from your site go through the api. If you have some extension that sends the mail using smtp, then the user is still going to get an e-mail. So, I've selected to unsubscribe, but I got a mail from your site...
If you give me reasonable explanation why merge tags can be usefull, I'll consider them in the next update.