This is a great tool, and I very much want to figure out how to use it, to better reach users of our own site.
Thing is, most registered users signed up mainly to participate in some particular site. Sending monthly emailed newsletters to them about Wikidot results in unwanted email spam. Sure, it's possible to unsubscribe from the newsletters. But it's more likely that many users will simply label the newsletter emails as spam, which causes more email filters kick in to exclude Wikidot, I imagine.
Really, this newsletter content is focused on wiki admins and moderators — the folks who are most actively building the site. Default newsletter settings for them should be "on", for everyone else, the default setting should be "off". If "off" isn't to the team's liking, another (somewhat less spammy) alternative might be to mail newsletters as PMs.
But the functionality is tremendous. Email broadcasting is fundamental to building wiki participation. But it needs to be focused and moderated, otherwise users tune it out and turn it off — potentially to the detriment of everyone.