Comment 3 for bug 1064822

Having a single Keystone service for multiple regions allows for a simple single sign-on type service for multiple regions. This, of course, is not the best way to do federated/single sign-on, but it's relatively easy to create at the moment.

The main use-case for this is if all users of your cloud use multiple regions. Rather than having to ensure all Keystone services in all regions are synchronized, everything just authenticates off of one.