Email (smtp+ssl, pop3+ssl, imap+ssl, greylisting + spam and virus scanning, virtual accounts authenticating against some central source like ldap) central authentication (windows domain, kerberos domain, something along those lines that can authenticate windows clients) file servers (again, windows clients, using central authentication) web servers (just simple static file serving and proxying dynamic content to app servers) database server (no postgresql for plan 9 still) app servers (pretty much no languages support plan 9, so our custom web apps are a no go right?)
As far as I know, I can't do any of those things with plan 9. Am I mistaken? Is there something I am not thinking of that I could use it for? Do people only use it in their own private little plan 9 only networks? It doesn't make much sense to offer plan 9 as an alternative to unix-like systems if it can't do 99.9% of the things unix-like systems are used for.