What about KDE or Gnome is so important that you would need the latest release or would stop you from using any of the backporting services that exist?Ever tried compiling all of KDE?
The point remains that any update to a central part of the system will force you to build everything that depends on that library from source. It's not just "building one application from source", it's building half of your system from source.
The power of Debian is in the high quality package repositories. The power of Debian stable is that the packages do not break things when you install them, which makes it attractive for servers and other situations where stability is important. The more you rely on the provided packages, the more stable and comfortable it is to use. You trade the bleeding-edge software for stability and maintainability.
Installing binary blobs into your kernel and compiling random stuff from source (and recompiling it over and over every time there is a security update) defeats the purpose of Debian stable, IMHO.
If it rocks your boat, then have fun doing it.