Getting pissy about it doesn't change the fact that the Hurd serves no useful purpose and doesn't have an interesting idea animating it, just a series of shop-worn ideas from 2 decades ago.
If I spent my off-hours building, say, a slightly different clone of Java that I'd started in 1996, you'd be entitled to tell me that I was wasting my time. In fact, this would be doing me a service. If I couldn't turn around and say, "well, Onanva is different from Java because of interesting features X, Y and Z" or "Onanva is about to take over from Java because I've got 100K users in beta and they're screaming for a commercial version" or whatever, then I'd have some serious introspection to do. What I wouldn't do is go into a screaming frenzy that someone who is "sitting on their ass" (whatever that means) is disparaging my precious, precious work and start running around with my dress over my head.
I can't tell from your profiles whether you or Mr P9 have ever been in an environment where you get asked tough questions, but any decent CS school will make you run the "why is this interesting/useful" gauntlet repeatedly, especially if you want to get a PhD or something silly like that. The bulk of the cruel, cruel people who ask you these terrible questions will do so from a seated position, and it will be imperative on you to harden up and supply answers. Not every hobbyist has to deal with this sort of thing, but not everyone outside industry or academic research is incapable of doing so; see also Young Torvalds vs. Tanenbaum for a less pathetic approach to criticism.
Programmers are free to do whatever they want; I doubt that a better OS will have much to do with the "betterment of society" in any case (you're not sure what I'm suggesting, but you're sure it's "quite offensive"). IMO they should work on projects that are objectively interesting. It's ok to disagree on what that might be, but no-one has been able to furnish a reason why building a UNIX clone on top of Mach (again, after cycling through several other underlying microkernels) is in fact interesting in 2011.