Having come from an Oracle background (6 years Forms + Reports + PL/SQL) into PHP (the last 10 years) I feel as though I should empathise - but I don't! This is mainly because I never understood why anyone would want to mirror application users as schemas in the database.
The big difference, I think, is that the database is generally regarded in the web development world a base for storing data. In the Oracle world the database is seen as the centre of the application architecture.
If you view user access, their rights and privileges as just data, then it starts to make sense that the business of evaluating the rights and privileges and acting accordingly is the role of the application. This is contrary to the Oracle view that the architecture itself should deal with it.
A huge advantage of managing the users as data yourself is the flexibility it affords you.
For example, what if you wanted a multi-tenanted application (paid for service, multiple business, all accessing the same database instance, but with data split so that each can't see the other's).
In the Oracle world you describe you would only be allowed to have one instance of a given username across the whole of your offering - I.E. between businesses. This sounds unmanageable.
And what if you wanted to allow usernames with spaces in them? Or use their e-mail address? You need to start applying some form of translation.
In my opinion, managing it yourself simply gives you more flexibility, and would be my default position.