John,
I shoot mostly landscape and macro with a Mamiya system, and have been since the 80's. This is why I have not invested int he limited lens. If I did not shoot the 645, I would think that a 12-24 zoom, 31, 43 and 77 limited would be about a perfect kit. Add in a good 200 and some long prime glass in the 500 range with a good TC, and I think a man might be able to wander off into the woods and just stay there.
My foray into digital came because I wanted to shoot some wildlife and birds now and then.
The sad reality is I didn't do enough homework when I did buy a digital camera. I looked at exactly 3 - the K200, the K20, and the Canon 40D. I chose the K200 because just like in my 35mm film days, I wanted a good basic camera. Live view, FPS, and other things were of little importance to me. I bought the camera as a body only. I had some old Pentax lens, and I purchased the DA 55-300 thinking the crop factor giving a perceived 450 mm view would be a good start. For most of the larger animals, moose, deer, bear, even coyotes and fox, that worked OK. The lens wasn't all that useful early and late in the day however, so I started looking for longer faster glass, which we know, does not readily exist for Pentax shooters.
I went to the K20 for the better ISO and more pixels that I thought would be better for cropping, which I was having to do a lot with the 55-300. But the lens is not up to the quality of the sensor in the K20d. Even stopped down, it is marginal.
I finally settled on a Sigma 150-500. as I could not find anything better. The bigma at that time was discontinued pending the updated version, and I was unaware of the Sigma 500 F4.5 The 150 500 is very nice at F9. But the shutter speed needs to be up there as it has no OS capability, and the K20 was not much good over ISO 800 so again, early and late day shooting were not very fruitful.
This strange journey eventually brought me to the KX with the higher ISO capability. I even made a post on this forum about that possibility. After a couple weeks with the KX though, I found that the mirror slap was a big issue with the Sigma. You can actually see the lens move when shooting the KX on it.
In addition, the Kx did not have the K200 or K20 feel to it.
Now you made a statement that today's digital cameras are so good that you would have a hard time picking one over the other from a bunch of A4 prints. While I would agree with you on this at base ISO, I would disagree with you at higher ISO's. But more important than the camera, is the lens. Yes the sigma 150 500 is very sharp at F9, it is still, F9, which means your background is not as pleasing as it might be at 5.6 or even 6.3, and the lack of sharpness there is very evident not only in prints, but on screen as well.
Right now, to do what I want to do with Digital Photography I see four options.
1) buy a Sigma 300 2.8 for 3 grand and use a TC.
2) buy a Sigma 500 F 4.5 for $4,600 and maybe a TC
3) buy into a new system all together at about the same price of the 500 4.5 and get higher ISO capability to boot.
4) wait till September and see what shakes out of the Hoya/Pentax tree, and perhaps be in exactly the same position I am in now.
If I thought for a minute that Hoya would allow Pentax to produce a K7 boxy with KX sensor, a moderately fast prime in the 400 to 500 range with a TC possible, it would be a very easy decision. But Pentax has already said they have no intention of making a TC, so........
I just dont see Hoya moving in the direction I want to take my photography. I'm not the only one who thinks this way, you should go read Ben's last few post in the New Pentax body thread in this same forum. I think you will get a good perspective of a Pro's view on the subject, and it doesn't differ that much from mine.