You don't have to be a member to take advantage of this. I syndicate my RSS feed to my micro.blog account (no membership) and have had quite a few useful conversations through there. I didn't see any point at first, but I'm sold now - it's a small community where you get to know people, which is rather rare at the moment.
API feeds is to pull data from Micro.blog - for example replies. To enable cross posting from your existing blog, a setting can be enabled. From the help page on getting started [1]
> "If you already have a blog, such as one based on WordPress, connect it to Micro.blog by adding the feed URL to your account under Account → “Edit Feeds”."