Just making an analogy:
JMS Wrapper <-> Core Api
is analog as
JPA <-> Hibernate
I don't see the JMS as another subsystem. That's a wrapper in top of HornetQ core.
The add-ons are basically JNDI bindings and other things like that.
For the future of the project, and how message systems are evolving, I think it's
better to separate JMS and Core. For instance we may have stomp clients, maybe AMQP some
day... and etc.
Who knows... JMS 2 is coming.. maybe we will need extra configs for JMS2 based on the
outcome of what's out of the JSR. (That's off course just a speculation since JMS2
JSR is still assembling the Expert Group).