MOOCs can help teacher PD by giving the teacher experience of what it's like to study a fully online interactive course. If we're keen to develop or being encouraged to facilitate courses online, we should know what it feels like to be a student in such a course.
I completed an assessed Mooc on uses of Moodle for teaching and got my first badge. It was a salutary experience - lots of challenges because of time zones, keeping up with the course as well as my workload, adding discussion forum comments and questions which either didn't get any response or got taken the wrong way etc. It was interesting to see what an emotional roller coaster it was is some way. There were some frustrations with having a problem and waiting (in my case, impatiently) for someone to solve it.
But there were lots of pluses - using well thought out and well-paced learning materials, reading or viewing some of the additional resources, chatting and learning from keen learners and helpers from all around the world, getting peer feedback, responding to comments and the sense of figuring out a lot of aspects on my own.
It was fantastic professional development because I could see the potential and the pitfalls and had the chance to reflect on my own responses. It's made a difference to the ways I work with students online and the expectations I have about capability for autonomy. Since this MOOC I've embarked on other MOOCs but only completed one other non-assessed one. Some become very boring because they follow a very predictable pattern of videos (sometimes just talking heads) and quizzes and not much else. I learnt about sequencing learning, what might be the features of a good MOOC and what students might enjoy or get frustrated with. If you want to do this I'd suggest choosing a MOOC that focuses on something that you need or want to know about, not a topic you're already fairly expert in.