Indeed, the naming is stupid, but Quartz Extreme and Quartz 2D Extreme are entirely different beasts.
And there's really nothing mysterious about how one or the other is enabled or disabled. All you've got to do is this:
1. Install the Developer Tools that came with your Tiger package.
2. Open the Developer folder at the top level of your hard drive, then Applications, then Performance Tools. In there will be an app called "Quartz Debug". Launch it.
3. Aside from some options to help make your apps draw faster to the screen, have a look at the app's icon in the Dock. If you haven't messed with your settings, you will notice that it says "QE" in green. That means that Quartz Extreme is available and running, like its supposed to be. And that's it, regardless of which version of Tiger you're running, at least as of 10.4.3.
4. Now have a look at the Tools menu--aside from an option to disable Quartz Extreme (obviously for debugging purposes, but you can try turning it off and see how things slow down), you will notice an "Enable Quartz 2D Extreme" option there. It is NOT on by default in any verison of 10.4 so far, but if you enable it you'll see a little "Q2DE" appear on the app's icon, and things may run a little faster. They might also be less stable, or you could see graphical glitches--this is why Apple has not enabled it yet.
10.4.3 has not changed this process at all--it was never on by default, and you can enable it just like before. All 10.4.3 did was turn it off if you'd already used that app to enable it for some reason, probably so Apple doesn't have to deal with complaints because somebody's kid turned Q2DE on without them noticing, and they're confusing problems caused by an experimental feature with new problems in 10.4.3. Or who knows, maybe it crashed the installer or something.
Try it if you want to know what the big deal is--I didn't see much speedup personally, but perhaps on slower computers or certain operations it helps.