I think I did say it was my personal take, based on my personal situation (YMMV)
I do want to comment on some of these though (I'm abbreviating you post)...
1) I must admit my uses are both stationary and mobile at the moment. I have a 5,1 Mac Pro to take care of the stationary stuff, a 2015 MBPr (15") as my mobile workstation and a 2014 MBA as my traveling companion. The last being basically my replacement for my venerable 12" PBG4. The problem, as I see it, is that while many of the mobile tasks could technically be handled by the powerbook, even though not all, the one thing the Powerbook does not and can never have, is the Battery life of the MBA.
2) That's a good one, but sadly (in my cases, i.e. travel and folks) quite useless, as that de-facto necessitates constant access to a newer mac (The one doing the syncing and sharing).
3) Here, I guess, our uses are too different to allow comparison.
4) Compatability with newer machines' results. Apple has been at the forefront of rapid obsolescence. My main problem whenever I've been using my Powerbook, even my machines still running 10.6 is that I'm starting to have trouble with compatibility with stuff created on newer mac's
5) and final paragraphs. I think the main difference between you and me is in that "how hard one is willing to work to make it work". I do stuff (for instance yesterday I was doing a lot of OCR stuff) which does tax computers. I waited until I was at home, to use the effortless Oomph my MP affords. The actual duration difference between even my MBA and MBPr is on the order of 150%. On a G5 (not to speak of a G4), those tasks would still not be finished. I use computers to be able to do stuff - to get things done, and not in order to either wait or to be able to tinker and invent workarounds. As long as I am the bottleneck in my man-machine combination, I'm happy. In that I'm an adherent of the "It just works" -school.
That does not mean I do not respect you, or the effort you expend in order to tweak and invent workarounds, but it does mean that that does not work for me.
RGDS,