Absolutely not. In fact, at my organization, I just used the StackExchange network as an example of a bad implementation of a user account system. I understand why it's implemented the way it is (due to the network originating as an outgrowth of SO), but for the user, it is very confusing.
A registered user of one site should absolutely be recognized as a network user on another site. While it is acceptable to have the user create a site-specific sub-account, this should be an easy process for already logged-in users. When the user goes to post a question, vote, or post an answer, they should get a lightbox popup with a customized message, e.g.:
Hey Daniel Newman (not you?),
Welcome to UX.StackExchange.com. To post, vote, and comment here, you need to create a site specific account -- but don't worry; it's easy!
Create an account based on my Stack Exchange network account »
or
Sign out of my network account and create a new account »
(This would need to be wordsmithed to be implemented, but you get the idea.)
To develop a fully-fleshed out solution, I'd have to be privy to the details of the SE network's ID system implementation. It's be really easy to screw it up otherwise. Here's one of my favorite stories of a single-sign on system gone wrong (from UXMatters).
