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I don't think Arch Linux is very advanced as a distro development hub. For instance, I checked out Archie, a LiveCD based on Arch, and it's pretty obsolete compared to the packages of Arch. I believe that Ubuntu can leverage the development borrowed from the community much better than Arch, because Ubuntu can and produce ISO CDs every day or so, including the LiveCD versions.
These distros that have nothing to lose do get some traction, but unless they go the extra mile and make their development straightforward, I think Ubuntu and Debian will appeal to many more distro builders and users.