One argument might be that it is battle-tested in academic use: We have been using Discourse as a discussion tool for lectures and courses for multiple years now, and our experience has been almost entirely positive. (The only real pain points have been in handling incoming emails from external users, which is a pretty advanced use and a relatively new feature.)
These courses had up to 500 students, and Discourse seems to have been liked by the students. Without having concrete numbers, I’d say that engagement was noticeably higher than with the phpBB-based boards we used before. 
Another argument is extensibility. For one course, we have a web-based tool that students are expected to use. It was pretty easy to develop a small plugin so file sharing links to this web platform integrate nicely into Discourse. It looks like this (but is interactive):
This is especially true if you are working with Ruby on Rails anyways