Actually, while the lack of anything but a rare need to reinstall OS X is a good thing for OS X users, it doesn't make OS X stand out all that much. It just points out the crapticity of Windows if the various Windows OS's have tended to need that just to keep working over time.
Everything that's been said here of MacOS X is pretty much equally true of MacOS 9, MacOS 8, System 7, System 6, umm, and back before that you're mainly back in the world of booting from floppy disk.
a) System 6 and the first iteration of System 7 were still freeware to Mac users. Schools with Mac computer labs would take the installation diskettes to one Mac, install the OS, tweak the settings, add/remove fonts and disk accessories (remember those?), set up the default printer and AppleTalk configurations, and then copy the resultant System Folder to an external SCSI device (or put it on a System 6 file server, System 6 did have file sharing but it monopolized the machine to deploy it). Every other Mac in the Mac lab would acquire their copy of that System Folder via plain-old drag-n-drop Finder copy to their hard drives. Then the students would be sold or (depending on school budget) given floppies wtih the new OS on them, and/or would copy the System Folder from the hard drive to their floppies. Then they would copy it to their dorm or home Macs if they had them. And let their friends make copies. And when they wanted to create a new bootable volume (same machine or different machine) they would make copies. So not only did any single Mac hardly ever need a reinstall of the OS, a single install of the OS would often be deployed on thousands of machines, gradually diverging as people added or subtracted custom fonts and disk accessories, printer drivers, and custom INITs (extensions) and cDevs (control panels). If you had posted the queston of the OP to folks from this era, I don't think many of them would have even understood the question. "Reinstall? Huh?" Probably fewer than one Mac user in 100 had ever even seen the Mac System being installed from virgin installation media and just assumed every system version migrated from floppy copy to floppy copy, moving from Cupertino to all points of the globe.
b) During the era between the intro of System 7 and the debut of the Power Macintosh, two things happened: you had to pay for the Macintosh operating system (although it would of course be bundled with your new Mac when you bought one), and, as hard drives became ubiquitous and large, the default contents of the System Folder sprawled out from ~5 megabytes to several hundred MB. As a consequence, far more people did their original system install from installation media. Still, for any given person, it was typical that to create a new bootable volume you would copy your existing System Folder to the new volume. Given the plethora of 3rd-party extensions and control panels, and the wide ranger of user preferences, it was just so much more convenient than installing from scratch and copying in all that chazarai. So again, a single installation from installation media would often suffice for several bootable volumes, so the final ratio of installations & reinstallations / # of bootable volumes would still generally be considerably less than 1.
c) The late System 7 era (the Power Mac era) coincided with the popularity of the Zip disk, and MacOS 8-9 with the dawn of the era of the cd-burner, so if anything the tendency of Mac owners to make easy copies of their post-customization System Folders and deploy them again elsewhere grew, rather than shrank, especially among the decently tech-savvy, those most likely to own (or be responsible for) more than one Mac. On the other hand, the OS had by this time finally become cumbersome and complicated (athough still elegant and simple by comparison to MSDOS, let alone Windows or Unix), so folks were probably more inclined to nuke their System Folders than to trouble-shoot weird errors and conflicts than they were in the smaller-system days. Depending on whether you consider booting from a Zip or CDROM and replacing the hard disk's System Folder in its entirety via Finder copy as "reinstalling", you could consider the ration of (re)installs to bootable volumes to have gone up in this era, or not. I'd say you'd get a number less than 2 over the lifetime of any full-numbered operating system (MacOS 8, MacOS 9) on any given machine no matter how you counted it though.
All throughout the pre-MacOS X days, we Mac users would hear PC users talk about reinstalling their operating systems every 6 months or annually or whatever to cure some kind of weird PC falloff in performance, and we'd laugh at them and say "Get a Mac".
It's not a new thing with OS X that we're free from that.
