This is getting a bit off-topic for -backports, but from my experience,
as testing gets further away from stable one has to pull more and more
packages from testing to satisfy dependencies. Then it's just a matter
of time until you hit a partial-upgrade situation that is not supported
by the maintainers and breaks your system.
Exactly... the IRC bots have a couple of factoids on the subject:
Yes it is possible in theory and often without issues especially if you know debian very well and know what you're doing, but it does rely on the maintainers having specified the exact packages and versions they require to get the necessary dependancies, and even then there may be differing behaviour between the releases that affect the preinst, postinst etc scripts.
There were also a lot of people in #debian with broken systems when Wheezy was released because they were mixing 'squeeze' and 'stable' APT sources and had done an upgrade, which had left them with a system that was part-squeeze and part-wheezy but not really either one. Generally completing a dist-upgrade to Wheezy should fix those issues though.
Anyway... I agree this has gone offtopic. It was only intended as a one-liner personal opinion.