Manton says owning your domain so you can move your content without breaking URLs is owning your content, whereas I believe if your content still lives on someone else's server, and requires them to run the server and run their code so you can access your content, it's not really yours at all, as they could remove your access at any time.
This is a slippery slope problem, but people are certainly capable of taking positions along a broad spectrum here.
The one thing I might worry about--particularly given micro.blog's--size is the relative bus factor of one represented by Manton himself. If something were to happen to him, what recourse has he built into make sure that people could export their data easily and leave the service if the worst were to come to happen? Is that documented somewhere?
Aside from this the service has one of the most reasonable turn-key solutions for domain and data ownership I've seen out there without running all of your own infrastructure.