Also, you can compare the brand name 'OS X' to 'Windows'. There have been six versions of Windows, but none of this matters as it's just naming.
The only difference is the way in which these products are sold, and Microsoft are moving to the Apple model, whereby product updates are released more frequently and build upon the established core, as opposed to rewriting an entire OS every 5 or 6 years.
Furthermore, the major releases of OS X have their own service packs as it is, and they're released a lot more frequently to keep up to date with changes in the product line, security issues and other improvements to the software bundled. There were eleven of these released for Tiger and the latter few were pretty substantial. Just like SP1, 2 and 3 in XP and SP1 in Vista.
You're just trying to justify paying to update your Mac.
Semantics lesson:
Updates - fixes and patches that are applied to the already paid-for software.
Upgrades - Completely new and individual versions of existing software.
Updates are normally free. Upgrades are rarely free with commercial software.
Updates fix the existing software, they don't typically add big new features. Upgrades do.
Anuerin is not paying to update his Mac, but he is paying to upgrade it. There is nothing wrong in justifying this. Unless you advocate piracy, because Windows isn't free either.
Addendum: usual suspects start Mac vs PC bullshit again. strawberry float off with the elitism.