I am really wondering why the SE community of sites is each under it's own login system --> to me it looks like it passes the session data to each but I am not sure.
The reason why I am asking is that I run a series of internal websites at my company. Right now they are all their own entity and use an SSO service. It is the same user population. I am wondering what are the plus and minuses of having the users go through the sign up process that they have on SE. I can see from a management process it would help me with rights based stuff but not sure each user would want to go through that for a small site.
Note: The question used SE as an example because that is one way I was proposing a solution to my UX. I would like answers on my problem not SE's problem.