OS 8's real usefulness is pretty narrow:

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Poppycock!

Apart from beachycove's excellent summary, and HFS+, OS 8 was the first version truly stable on PPC. My memory of introducing 8.1 to my 6100 was that suddenly it was able to run for more than five minutes without bombing.

Aesthetically, I found the clean modern look much more appealing and useable than that of any version of System 7. And that look stayed with the Mac OS until the introduction of OS X.

- Only one version is really of any use at this point: 8.6
- It's particularly well-suited only to a narrow range of Macs made in only two years - 1996 and 1997

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As others have pointed out, 8.1 gives a relatively modern UI and capabilities to '040s and up, so the year range is quite a bit wider than the two years you claim. With a bit of tweaking, it can run on '030s, up to Beige G3s and iMacs. It would be ideal for a kerb-score iMac if you couldn't be bothered upgrading the RAM. And the improved networking layer makes Road Apple x2xx and x3xx Macs passably useable.

http://en.wikipedia.org/wiki/Mac_OS_8

largest overhaul of the Mac OS since the release of System 7 / an effort to integrate many of the technologies developed for / Copland. / Mac OS 8 is one of Apple's most successful software releases, selling over 1.2 million copies in the first two weeks.
/ Platinum interface and a native PowerPC multi-threaded Finder. / HFS Plus / PowerPC native versions of QuickDraw and AppleScript / Sherlock

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See also: Mac OS 8 and 8.1: Maximum Size, Maximum Convenience

even a fast 603ev is a dog running 9.x, whereas it runs any version of 8 very well;- my PowerBook 2400c likes 8.6 a lot.

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My 1400s like 8.6, too, but it feels a little heavy going - I was considering dropping them back to 8.1.

Which brings up another point - it's hard or impossible to find and/or fit enough RAM to early PPC Powerbooks to run OS 9 comfortably. This makes 8.x an ideal option.