You could use those interfaces outside an MBeanServer and do whatever you want.
However, the typical JMX usage is to go through the MBeanServer when registering for Notifications. In this regard:
a) Non MBean listeners can receive notifications from MBeans (i.e. listener can be anyone)
b) MBeans can only receive notifications from other MBeans (because they'll make their registrations through the MBean server, and the MBeanServer can only register MBeans)