Jusy my view:
The biggest advantage of a seperate login DB, I guess, is that all your users and roles are centralised in one place. This also gives you the potential to use or migrate to other A/A technologies such as Active Directory.
From a security point of view, this enables you to 'lock down' all your client DB's by potentially implementing a single user ID/password to access client DB. Connection pooling works great when the connection string is 'static', a performance bonus.
Client DB's will only consist of 'application data' and could therefore simplify recovery when there is disaster.
I am sure there are also a few disadvantages as well...