(1) Being "already set up for testing" for an older OS version
and
(2) continuing to use the testing setup for that version in addition to ever increasing new test setups (for 10.6, 10.7. 10.8 etc) and codepaths
is not the same thing.
As you get newer OSs to support, maintaining older testing setups means more work, and more IFDEFS and such in the code.
Putting more care in the LATEST offering. Putting more care in older versions if a loosing proposition.
And it's not just a "throw more people / money at it" problem. After some point the complexity of legacy support grows exponentially and the code suffers (or gets hold back).
>Car makers don't stop making spare parts a couple of years after a new model comes out. If they did no one would buy them.
5 years in computing is like 20 car years...