I hate to reignite an old thread, but this is one of the top results on Google, and I wanted to throw in my two cents on the matter. It seems like the defense of using subdomains over subdirectories has centered around SEO, but I think a case can be made for subdirectories regardless of SEO.
First, a lack of support for subdirectories seems like a deviation from a longstanding convention. Discourse’s subdomain approach is inconsistent with nearly all other discussion software I’ve worked with in the past.
A big thing for me is branding. At my company, I have our website using no subdomains at all, not even for our CDN, and especially not WWW. All resources branch from the naked domain, something I’d consider to be a best practice, or at least a goal to strive for, for customer-facing websites.
Encryption is another issue, and this does actually speak to SEO. With Discourse installing to a subdomain, most websites would end up needing two certificates or one wildcard certificate; both of which would not be necessary if Discourse installed to a subdirectory.
I’m looking into using Nginx or Varnish to rewrite the URLs in this manner. Not sure where I’ll get with that, but I’m optimistic. Any plans to support subdirectories in the future?