Now that backports is part of Debian, I think most of the issues with "out of date" packages are much less painful than they used to be. I suppose factoring main into "core" and "non-core", would make maintaining backports easier too - you could then test building a package against a smaller set of core packages and libs.

Packages needing newer versions of glibc, automake/autoconf, gcc/clang/ruby/python/perl etc are still tricky though.