IMHO in most cases when user creates a new project and 'Add to existing workspace' is selected - it is reasonable to make the project active, because most probably the user will want to do something with this new empty project immediately.
Currently (6.0 RC2) if there are some projects in workspace already - another project remains active, which is confusing (until you learn this behaviour) because all menus continue to refer to the old active project, not the one just created.