Goodness, I checked this thread out when it started and then basically let it go, because I agreed with David's basic points but felt they don't apply much if at all to me. Then I noticed it had taken on a life of its own so I just checked back and read through it. And I'm left with two overwhelming impressions.
First, I have absolutely no doubt in the world that pixel peeping is useful and necessary for some types of photographers and particularly those who make a living at it.
Second, I'm overwhelmingly happy that I'm not the type of photographer that has to think about it.
Because frankly it makes my head hurt. I had a good career in a field that required a lot of detailed technical knowledge and I acquired and put that technical knowledge to use every day. Photography is something I enjoyed immensely before having a career and family and now am enjoying even more as I've transitioned into a mostly consulting role in both career and family (my kids are out of the house and they ask questions when they have 'em and I'm happy to be of service, but I'm not in charge in any way, shape, or form anymore and bear no further responsibility for the outcomes - the classic definition of a consultant). But I've never had to please anyone but myself and a couple of college professors with my work. Its always been all about the image to me - I've never had to be concerned about the technical parameter of the outcome. I've taken some of my favorite images with cameras that were technical garbage. And I've seen images produced by some brilliant photographers with Polaroids and Instamatics and other crap cameras that could still be used to create great images. Since I got back into it again a year or so ago, even the lowliest Canon S90 (not lowly, just the lowliest I've owned) has been capable of producing what I consider overwhelmingly good photographs and have not hindered the images I could make. I've never sold a photograph. On the off-chance that I ever do, its gonna have to be on a take it or leave it basis - if someone likes it enough to buy, they're gonna have to like it technical limits and all.
I occasionally print up to 11x14, and have printed up to 20x30 just to see what it looked like, but usually my photos only come off of the computer screen to go into a photo book at no larger than 8x10. Any camera I've used can produce better than adequate results for this. Hence, I don't care. I'm not saying others shouldn't care or that many don't HAVE TO care. I'm just saying I don't care. And I'm damn pleased not to have to. So far, the sum-total of my gear testing was taking a few brick wall shots at different apertures of two lenses that I'd read had really lousy quality control and there were plenty of good ones and plenty of bad ones made. I did these tests to make sure I didn't get a bad one - and to the extent I could tell, I didn't get a bad one. And if I did but couldn't tell, I guess it didn't matter. I sometimes look at a 100% crop from the X100 just because they look so damn good, but ultimately when it comes down to making photographs, I don't look at 'em that way.
I'm not claiming my non-attention to pixels makes me superior or inferior to anyone else. I'm just noting it and enjoying it!
Oh, as for the analogy of the guitar having two strings, I think that's a somewhat silly analogy (although Keith Richards DID reinvent rhythm guitar with one having five). To me, there are basics that a guitar must have (six strings, a bridge, a nut, tuning knobs and some form of amplfication, whether acoustic or electric) just like a camera must have a lens (or a VERY tiny aperture) and some sort of sensor or film and a way to actuate it. You wouldn't take off the tuning pegs or the strings and claim it was still fine any more than you'd remove the shutter button from the camera. BUT, a Mosrite electric was a cheap piece of garbage when it was made and a Gibson Les Paul was a masterpiece coming from the factory yet there were still types of music and applications where the Mosrite and a cheap, tinny amp sounded better than the Gibson and a stack of Marshalls. So leave the strings on the camera, but different tools for different jobs...
-Ray