The time the legal system takes to settle a case is another issue as is the reality that anyone can argue anything.
What is commercial and non-commercial has to be judged on each specific case's merit, which obviously makes general discussions difficult beyond broad definitions, but in general it is not a big grey area.
The grey area is massive. Take using photos. What if I run ads or have affiliate links on my blog? What if I'm a US not-for-profit that has $10s of millions in revenue? What if I'm giving a presentation at an industry event that I'm not being directly paid for but it's essentially part of my job?
I could go on but I guarantee that if you surveyed the readers of this site many people would answer those questions differently.
Effectively, if a lawyer were making the call of whether a particular use was OK, they'd tell you just not to use it because no use that you'd be asking a lawyer about is clearly OK.
Well, see, I'm reasonably comfortable with using NC so long as I'm not selling or being directly paid for something. Otherwise, pretty much everything I have in public is associated with my job at a commercial enterprise or other business in some manner and I effectively can't use NC for anything non-trivial. So, without surveying the rest of HN, we already disagree on a wide swath of use and probably wouldn't change each other's minds.
What is commercial and non-commercial has to be judged on each specific case's merit, which obviously makes general discussions difficult beyond broad definitions, but in general it is not a big grey area.