The answer probably lies at several levels:

1. The Army is enamored with outsiders
2. There is also a liaison function between Army & State (which we've both done as Army guys, but their guy somehow has more street cred with our uniformed bosses)
3. It keeps State pregnant with the small wars issue

POLADs & PRTs also appear to be redefining how FSOs see their functions. I served with a coupla Vietnam Era CORDS grads who were very activist in their outlook on their role in the host nation. The following generation appeared to me to be much more constrained. They wanted to conduct structured meetings with appropriate officials and write cute cables back to feed the info monster in DC. I'm getting anecdotal evidence now that some of the POLAD & PRT FSOs are now buying back into the activist role.

That said, there is still a lot of irony in the fact that State hires vets right out of the service on short term bases to fill some of their deployed commitments. Where is the civilian expertise in that deal?