It's an interesting ethical question as to whether a profit-making business should be allowed to use willing volunteers as part of its business model.
Yes, you can argue that, if there is a job to be done, then it should be a paid role because this helps to create employment for someone who needs a job.
However, employing night moderators would take away the option of doing it on a voluntary basis for the volunteers who are currently choosing to spend their time doing this work, presumably because they feel that they get something out of it. Many of them might not want to do it on a paid basis, so it would seem a shame to take that opportunity away from them.
On balance, I feel that MN uses volunteers instead of paid staff simply because it can, and I don't really see anything wrong with that, as long as the volunteers are free to do as much or as little as they like, and as long as there is transparency about the fact that MN is a profit making business, which it is. Arguably, we are all volunteers contributing to MN's profits, because we are all voluntarily producing content for the site, and it is the content that ultimately keeps them in business. Should all of us be paid to write for MN?