Creating and/or using backports on Debian stable is a way to keep up-to-date while still being on stable. However, doesn't using backports destabilize stable? If not, why aren't backports simply a part of the main stable repository?
However, doesn't using backports destabilize stable?
Not if you are careful and selective. I use backports all the time (I run Debian stable) and have never had a problem.
The key is to avoid upgrading or otherwise changing core packages. This includes the basic libraries that are widely used in the system, including important C and C++ libraries (e.g.
libstdc++), the init system, the default versions of important language implementations like Perl and Python which the base system depends on, etc. I've also avoided upgrading big complicated subsystems like KDE and GNOME, because it just too messy, though it is debatable whether they are part of the base system. You don't have to use them. On the other hand, many end-user packages in Debian do depend on KDE or GNOME, or at least their associated libraries.
If you restrict yourself to leaf packages, i.e. packages that no other package depends on, you will be fine. This is usually the category that most end-user applications belong to, anyway. Occasionally, it is Ok to backport more important packages, as long as one is careful. For example, I have backported TeX Live, which a number of packages do depend on. Another approach is to install non-default versions of packages alongside the existing default versions. For example, Debian supports multiple versions of Python, so one can upgrade or even add more recent versions of Python without compromising the integrity of the system. Similarly, one can backport newer versions of GCC because Debian supports multiple versions of GCC. It is generally also Ok to upgrade software like the kernel and drivers, because these are usually only weakly tied to user space.
NOTE: To be clear, in my remarks above I'm including self-created backports, not just the ones made available officially by Debian via backports.org. For example, to my knowledge, there has never been a TeX Live backport made available via backports.org, though it is actually relatively easy to backport.