I came to this post to see how others are doing allowing registrations to their blogs.
First, the general settings are under your control, uncheck "anyone can register" and they wont be able to. If you do want them to register and subscribe, check that box and choose "subscriber" in the drop down menu, that can allow people that want to subscribe to your blog to do so. I think they would do that to get links back to their site when they comment, you will moderate the comment and approve or delete it anyway.
Set the check box to "anyone can register" and the drop down to: author, and now you allow people to join your blogging community, that is what I am doing, this can be beneficial, Mike Liebner from Article Underground, literally has thousands posting to his blogs as authors it helps his blog show up on page one of search for many different keywords with their articles, if your blog gets popular well then, like Mike says "words equal money"
If you are blogging private then just uncheck that box and get the security plugin.
Still wondering how anyone else is doing allowing user author registrations, guess I can keep looking aroung.