It seems like most of the messaging systems I've looked at have basic, if any, support for priority message queues. For example, the AMQP only specifies a minimum of 2 priorities. RabbitMQ, an AMQP implementation, doesn't support any priorities. ActiveMQ will be getting support for 10 message priorities in version 5.4 in a couple days. 10 priority levels is the specified by the JMS spec.
A priority queue in the non-messaging sense of the word orders its contents based on an arbitrary field with an unconstrained range of priorities. Why does an implementation like this not exist as part of a messaging system? As I asked in the title, is priority an inherently non-messaging concept?
I realize that one answer might be that the concept of priority introduces the possibility of messages infinitely languishing in the queue while higher priority messages are processed. Are there other reasons?