I would NOT use testing in the sources.list because it can break immediately after wheezy final release. It is much better to use the codename like wheezy in there - i use wheezy since about 1 year ago because there it was known that it will be frozen soon already. When Debian is in (pre-)freeze state it is often already stable enough for many use cases, especially for desktop systems. The problem is always that the freeze can take very long - if you wait for final you wait definitely too long. I hope jessie will be a bit faster from freeze to release, as there shouldnt be so many transistions to do like from single to multiarch this time.
You seem very confused. Testing is where, as the name implies, 'testing' of new code is done, it's here (and of course upstream) that bugs are found and (hopefully) fixed.
Once these found bugs have been fixed and the code seems to work proper, it will be moved into 'stable'. So I seriously doubt that code from testing is 'usually' more stable than code from stable in Debian, it defies logic.
Do you have anything to back this up with, like some Debian developers telling people that if they want stability they should use 'testing'?
Basically, long fixed upstream in 3.8; but affects all previous "stable" systems.
So they went though patch backporting(actually a rewrite), which made into Debian much much later than in 3.8+.
When you use Debian for 3-4 years, it comes clear that whole stable/unstable paradigm does not work.
You can introduce more bugs, when you fix bugs. True.
.... But the ideal definition of "stable" is just to find current less-buggy state, several steps from cutting edge, and to give full power on actually fixing bugs in cutting edge.
Instead, versions are frozen, bugs are found which were already fixed by rewrite of something that is still not in frozen version; so the fix must be rewritten explicitly for frozen version - *puff* you have several versions to support and much more work to do.
That said, I am debian user if it comes to binary system. But I always like source-based much more, even if its more troublesome. But at least those troubles actually act in direction of working on cutting edge. Not patching of something that was long patched by another patch.