Actually it was Rafał who liked it! 
I think the choice of the distro for the AppVMs is of negligible
importance. Most of the desktop functionality (like the desktop
environment) is removed by Qubes anyway. So, it just comes down to the
pkg manager, specifically to the yum/rpm vs. apt/deb religious wars.
The choice of Dom0 distro is of more concern, and ideally we could have
a custom distro here (at least a forked one and maintained by us). But
the problem is that, at least currently, Dom0 also serves as a GUI
domain, and so it should have the latest Desktop Environment and Xorg
drivers to make the GUI look slick 
(And to support the latest GPUs).
In Qubes R3 we plan to split Dom0 into "the actual Dom0" AKA admin
domain, and the GUI domain. Then we could probably use some minimal,
custom distro for the Dom0/Admin domain, and some
whatever-your-preferences-are distro for the GUI domain. Secure,
reliable GPU passthrough is needed for this, of course.
joanna.