It would be much easier to work with the piwik.org services if we were not have to make a special account for each of them. Now the login-pwd pair of trac doesn't work in forums. Blogs must have their own auth.
The ability to use "social login" is realy needed and oAuth is one of the possible solutions.
I agree SSO would be easier, but given the problems with "social spam" in general (and ongoing spam in the forum and trac), oAuth is a wontfix.