I think the lack of OS 8-specific lovin' is because there's really nothing "special" about OS 8. OS 9 has Platinum, but it also has more built-in functionality to boot (especially hardware support), and was the last classic Mac OS. Conversely, while certain apps will only run in System 7, I don't know of anything that will run on OS 8 but won't run in OS 9, and System 7 has a distinct "classic" look some people prefer. System 6 needs no introduction to this group. 
The only reason 8.6 is in my rule of thumb is that it's smaller and works better on pre-G3 Power Macs, for those apps that don't need OS 9. And 8.1 even on a 40MHz '040 does not run extremely well, though it's acceptable on my Q800; 7.6.1 is a lot sprightlier.
If there were something about OS 8 that made it stand out, I think it would get more fans. But OS 8 gets more respect for its presence in Apple's history than its distinctiveness, and the lack of distinctiveness IMHO means it's fondly remembered but not particularly used.
The only reason 8.6 is in my rule of thumb is that it's smaller and works better on pre-G3 Power Macs, for those apps that don't need OS 9. And 8.1 even on a 40MHz '040 does not run extremely well, though it's acceptable on my Q800; 7.6.1 is a lot sprightlier.
If there were something about OS 8 that made it stand out, I think it would get more fans. But OS 8 gets more respect for its presence in Apple's history than its distinctiveness, and the lack of distinctiveness IMHO means it's fondly remembered but not particularly used.