No, it's not strictly mandatory. But if you don't upgrade, you won't be able to email your invoices from the MYOB system. You'll be forced to save the invoices to PDF, then attach and send them from your own email. So essentially, it's mandatory - unless you don't use MYOB for invoicing customers.
If you do upgrade, you must be very careful with the online invoice payments that are switched on automatically. You can change those settings, but it's complicated, and may not work if your invoices are generated through a 3rd party app, such as Service M8.
Good luck.