OK. I am going to take the lack of responses as a response to this question.
This kind of global questions are un-answerable on the forum.
Catch me at some conference - JBWorld is coming up in Februar ;-) - and I'll be glad to discuss this with you.
One benefit that comes to my mind is that Spring takes plain POJOs.
But then MBeans can also be POJOs, but they need additional information or must implement -MBean interface.
If your system changes a lot, then you need something more powerful than plain Spring beans - that's what JBoss JMX provides. Hot (re)deployment, finegrained lifecycle, real dependencies, ...
Or have a look at the new cool stuff we're doing - JBoss Microcontainer project --> the new JBoss5 Kernel.
Yes, you are right, it was a unfair question for the forum. Thanks for taking a stab at it and I will check out the items you have mentioned.