The mailchimp plugin was originally written to sort of gather all users emails, as there was no way to export them, and even before the emailer system was implemented. Currently, I think sending a welcome email via MailChimp is sort of redundant, as the NodeBB process, if you have an emailer plugin installed (such as Mandrill or Mailgun etc.) will send a verification email to each user, welcoming them and asking them to verify their email (don't quote me on that, try it)
Also now, there is a way to export users' emails, using the Export CSV options in the NodeBB admin panel. and I am aware that the team have a feature on their plate to support email-blast to groups and users. (Till then, if you want to blast emails, you can export your users into CSV format and use this that uses your same mandrill key to blast emails.)
The Mandrill plugin (or any other emailer, there is a few) is required if your NodeBB app needs to send emails, such as Verification/Welcome emails, Daily Digest, Password resets, etc..., I find this one to be a necessity.
Bottom line, you do not really need this mailchimp plugin, installing the mandrill one should be sufficient, but I will address the issue you're seeing tomorrow.