I understand the reason for having a backport is so that you can run newer software on an old system without using newer (and therefore unproven) libraries. That is, the newer software would get built by an older (and certified stable) toolchain. In the case of the Linux kernel (Debian 6 shipped with 2.6.32 and the backported kernel is 2.6.38), why is there even a backport? I ask because I suppose that the kernel doesn't have library dependencies, so installing a kernel from the development suites of Debian shouldn't cause issues.
The backport kernel is useful for people who have newer hardware requiring more recent drivers than what the stable release can offer.
Installing a kernel from unstable or testing on an installation of stable would be difficult: you'd have to add unstable or testing as a source, and write a proper preferences file to only get the kernel. And possibly run into trouble if the kernel packaging changed, or if the other software that the kernel packages interact with have changed (bootloaders, module compilation frameworks, possibly ABI incompatibilities in compilers, …).
From backports, you get a package source that you can just drop into