@sdubois it is certainly tricky getting an objective answer from me, since I am a co-founder of discourse.org.
I guess some big reasons why for a project the scale of Rails a solution like Discourse makes sense are:
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We get to keep mailing list mode and very rich notification fidelity
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We get to keep one home for all Rails discussions vs a tab on a series of Rails repos
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We get to maintain all our history including tons of very interesting topics like reasoning DHH had 14 years ago regarding building a migration system for Rails like: Migrations Vs SQL compare tools
I think beta discussions by GitHub is a pretty good fit for small projects looking to quickly bolt on discussion, but once you grow having branding, categorization, security, tagging, mailing list modes, oneboxes, moderation fidelity and so on are all pretty important considerations.