Pixel peeping might be useless, but so is photography generally speaking. It's totally natural, in a photographer, to dig into every little grain of a negative. It's even the leit Motiv of a film by Michelangelo Antonioni,
Blow-up. If we really were rational people, we probably wouldn't be doing photography at all. I suppose fishers can test and buy their canes and threads to support shark fishing and then using them for anchovies. It's human nature. We are fascinated by technical achievements, by the potential of the materials we use. We like technology. People who can buy Ferraris to drive to the newspaper kiosk because they are charmed by the potential, the technical content.
That said, I think the question is not properly asked. Final resolution is the result of, so to speak, the "product" of factors.
It's as if you had a "lens resolution factor" and a "film resolution factor" and you multiply the two to obtain the "total resolution product". If you keep one factor the same, and raise the other factor, the resulting product will be higher in any case (in support to post #27).
Regarding the Leica difference, or the Zeiss difference, what I read during my early young years (I'm still young but it's many, many years I've been young, so I say that to avoid misunderstandings

) is that it's not really resolution that makes the difference between good and bad lenses, it's more a question of acutance. Zeiss and Leica/Leitz lenses have/had a reputation of high acutance. Acutance is more difficult to obtain than resolution, but ultimately, for the human eye, acutance is a very important factor in the sharpness impression, in a sense more than resolution. Cheap and ultimately disappointing lenses can sport very high resolution figures. "Perceived sharpness" is the question, and "resolution" is not the answer.
As far as I know, there is no standard or objective measure for acutance. Resolution can be "measured", but acutance cannot. Sharpness being the combined effect of resolution and acutance, there is no objective or scientific measure for sharpness. The various tests that laboratories never cease creating never really manage to measure acutance.
Grain can increase the perception of sharpness because it can increase acutance.