You obviously have never worked in the workstation market in sales or support.
Actually, I have back in college, but that's been awhile, and the technology has long since changed. Fortunately, for the better these days (namely PCIe bus, rather than a succession that required new boards,... to keep up).
Because there are plenty of instances where 3 and 4 year old systems were unable to upgrade to the latest graphics hardware. This was common with SGI systems. There were several NT-based systems that could not be upgraded to newer hardware. Some because they did not have the correct slots and some because the correct drivers were not made available.
In some cases, particularly graphics workstations, yes prior to the introduction of the PCIe bus. The GPU bus kept changing often enough that the smartest thing to do was get a new system each time, which would have been required at ~3yr cycle. But that's no longer the case, as we're now on PCIe, and what my statement was based on. Beyond that, it's more determined by software and CPU requirements. As you can swap CPU's in some cases, that means the majority of the system can remain in service longer.
But like anything, it's all down to a case by case basis. But my point was, that with PCIe, there's more ability to retain systems that otherwise would have had to be re-purposed for another task (typically a different user that doesn't have as high a requirement for performance) or sold off/disposed.
Then there's workstation use that's not 3D intensive, such as what I do with it. Some 3D (layouts), but it's small overall, and I can get away with a decent consumer card. What interests me, is GPGPU use, as adding 2 or 3 inexpensive cards have the potential of making a drastic improvement, even over a DP system, as the software isn't capable of using all the cores in such a system. The software's too far behind. Even if it could, the GPGPU performance will be more than that a second CPU can provide anyway.
Apple will support the systems for at least 5 years. That does not mean those systems will get the latest and greatest hardware and software. That is unreasonable.
I look at it this way. Extended Apple care provides 3yrs from the date of purchase. So an '07 system would be ending now. That means before 2011 is done, the last of it will have completed (presuming there were '07's sold in 2008 either before the early '08 MP's shipped, or as a refurbished system). That's all that Apple really seems to care about. Any support afterwards seems to be by happenstance.
For example, the HD4870 can actually work in those systems (though it has certain limitations, namely GPGPU floating point calcs are only single precision). They could choose to sell it to EFI32 system owners with a proviso that it's not really suited to GPGPU use under SL. Rather easy to do, IMO. But they choose not to, and tell users that it won't work.
