According to the following page http://activemq.apache.org/activation-spec-properties.html
The clientId default is set by the resource adapter. This is not the case in the code.
When defining the MDB to be Durable via subscriptionDurability, if the clientId is not configured an validation error is thrown stating "clientId must be set since durable subscription was requested".
Looking at the code, if the documentation is correct, the ActiveMQActivationSpec validation method should no longer validate the clientId. The ActiveMQConnection makeConnection should change
String clientId = activationSpec.getClientId();
String clientId = defaultValue(activationSpec.getClientId(), getInfo().getClientid());