<i>Why do LMCE repos need to host the packages that the Ubuntu project is not only hosting, but also patching, with their larger, more focused staff? Why does a LMCE repo need to host any package other than LMCE-specific ones? The LMCE sources.list file can point at both the Ubuntu and LMCE repos.</i>
Say L-1.0 depends on U-1.0, and X depends on U-1.0 as of Nov 21st, 2007
Say the L-1.0 package is installed when LMCE is installed and is a major component of LMCE.
Now on Nov 22nd, someone tries to install X after setting up an Ubuntu source in the sources.list, but X now depends on U-2.0 and won't install without it. Since L-1.0 depends on U-1.0, it won't be installed. Or worse, the user might try to force the installation of X and break L-1.0. (The proper thing for the user to do would be to install an older version of X, but manually satisfying package dependencies quickly gets tiresome.)
By keeping our own mirror of the Ubuntu packages we can make sure that X won't be upgraded to the version requiring U-2.0, until L-2.0 which can use U-2.0 has been compiled and tested. We don't need to touch the Ubuntu packages themselves, just update the LMCE packages in concert with the Ubuntu package changes so that package dependencies can be satisfied.