I think the strongest criticism of this is that "GLBSE" is a legal target. If you had some decentralized P2P brokering, then the gov't would have to fire buckshot to hit everyone.
They couldn't just go after one person and do significant (though temporary) damage.
That said, I don't see any reason not to start with "GLBSE" and then figure out P2P brokering later. The gov't will take a dim view of either. P2P brokering will be more complex and take more time.
I suspect that in terms of the innovations' usefulness:
P2P Brokering > "GLBSE" with colored coins >>>>>>> GLBSE