However in exchange for that, I get real progress in the software, and when something inevitable go wrong, chances are someone else has had the problem because there's such a limited range of configuration options. As a professional developer it's pretty easy to justify the Apple tax on that basis alone.
They just ended that record with the Snow Leopard/Intel requirement, but that seems like a reasonable thing to do.
And your old PPC hardware runs fine with 10.5.
It's not quite that simple. I spend a fair amount of time writing OSX drivers for, erm, non-Jobsian hardware. With a little hackery (not even programming-level hackery these days), OSX can be remarkably stable on a wide range of system configurations.
Specifically, Apple seems to ship (and test?) drivers for a good deal more hardware than you'd expect. Point example: the 8800GT card that they ship with the Mac Pro--the driver actually works on any 8x00* card, as long as you can disable the check. The GMA950 driver works on a wide range of Intel cards. The ATL1e driver works on a lot of Atheros chipsets. AppleHDA works on a lot of sound cards (If your motherboard has an Intel chipset, it probably works with minimal hackery). Similar to the way Linux drivers work, OSX drivers tend to cover lots of similar hardware. So AppleHDA might replace 100 separate windows drivers for separate sound cards, and the big ones even do several chipset companies.
Lots of the hardware work (some of which makes it into mainline OSX) is done by the Darwin people. I've seen tons of straight-up ports from Linux (SpeedStep for non-Core2Duo processors, for example). There are lots more.
All of that to say, the OSX hardware landscape is far beyond the four-at-a-time macs that Apple actually ships. With respect to driver support, OSX isn't far behind Linux. I'd put OSX's superior reliability as a consequence of their architecture decisions rather than their (purportedly-lack-of) hardware support.
The real sunk cost with hardware support isn't driver development, (paying 4 grad students $400k) but paying people to answer the phones and troubleshoot the devices, paying FedEx to ship the things around, EETs to diagnose and desolder, somebody to write the technical documentation, Geniuses to take tests on the part numbers, etc. Guarantee Apple tests lots of hardware they never end up shipping because it can't clear the reliability hurdles; all those things need drivers.
In any case how many different drivers are out there for Windows Sound? How many are out there for Linux? Bash ALSA or OSS all you want, but they support a ton of hardware, port that sort of support to windows or mac and you got yourself ONE driver heavily tested by freaken' everyone which makes debugging easier with a central point of service.