What advantages are there to using 64-bit mode?

The main advantage is that you can address vast amounts of memory. With 64-bit mode, the application memory is not limited to 4GB as with 32-bit applications, so there is essentially no practical limit by today's standards. As a result, all the installed memory that is not needed by the OS is available for use by Logic Pro or MainStage and all the included plug-ins, which can be meaningful if your Mac has more than 4GB of memory installed. This larger amount of memory allows you to run far more instances of memory intensive plug-ins, such as sample-based software instruments.