That's not really true. "Suspended" is not the same as "Disabled". KWIN can be "enabled" and set to auto-suspend when a full screen application starts, and it works flawlessly so KWIN doesn't sap performance from your video card.
There are at least two major benefits that KWIN can do that can significantly increase your desktop experience on slower, older hardware.
The first one is "scale resize" effect. This allows you to GPU accelerate resize any window and it will resize the window with perfect smoothness. If you turn this off, resizing a firefox window with a modern webpage loaded inside it is very choppy on my PC. This desktop effect is *amazing* as the CPU usage while resizing windows drops to 0% instead of 100% and it is *very* smooth on a 6+ year old laptop.
The other major benefit that KWIN compositing can do is fix the repainting problems where you move a window in front of another window. The CPU is too busy to redraw the other window so you see a messy trail of the first window all over the second window. At least until the second window gets a chance to redraw. So you get presented with an incorrect display of your open applications. With KWIN Effects enabled, this simply never happens anymore and you can drag windows wherever you like without having any window trail problems.
So yes, even though I'm running kwin effects with only 1 desktop effect, I'd definitely say that my Desktop is a lot better because of it. Also, I can confidently say my Desktop is better than Windows XP as I know the "window trailing" in front of other windows that "aren't responding" was always an annoying graphical problem on Windows XP.
Further, I can force my GPU into a low power state and KWIN still runs smoothly.