Returning the last_insert_id is something every reasonable SQL database must do. It therefore makes sense to have a common interface for that, just as you have a common interface for opening a cursor and emitting a SELECT statement -- no matter how cursors are implemented internally or whether the database is a file or a network connection.
I have no problem with getting access to the raw functions. But that kind of implementation detail wants to be abstracted. It's not something a Web programmer should have to think about.