I think deep integration with MailChimp is a huge reason to use Mandrill. I'm not and have never been a paid MailChimp customer, but this might get me there.

I currently use MailGun for transactional e-mail, and CampaignMonitor for marketing e-mail. MailGun's UI is really rough, so Mandrill is pulling me in for that reason alone, and CampaignMonitor pretty great but I haven't use it lately any way.

Have you considered discounts for developers deploying individual accounts for clients?