Sorry if I have not kept up with "open Guild" but it sounds to me like a key component is one requires permission and the other doesn't.
Over the years, I have accumulated dozens of patches in various places and while I probably submitted some through Trac, if I could just fork a repo in an organized manner without asking anyone's permission, I could keep my patches in one place (irony?) and in sync with some ease. This benefits Boost because then my patches are not in multiple repositories behind different corporate firewalls, but somewhere accessible to others. Not that they are anything spectacular, mind you!
I think you *might* enable more contributions as a benefit of a decentralized process just based on my experience.
Well maybe, but how will developers ever know about these patches unless you tell them about them (presumably via an issue tracker)? Come to that, can't you do this right now with git-svn? Cheers, John.