Guy mentioned if two cameras were on a table he would take the one with the highest MP. He is not alone, for sure.
But's it's very interesting to hear what the inventor of CMOS sensors has to say on exactly this issue:
This is a long lecture at Yale I think, but you can just forward to 38 minutes where he talks about the problems with small pixels and how marketing is trumping engineering and physics with high mp cameras like the A7r2. It's really worth a listen as he basically says: high MP counts "sell", so we make them, despite the fact they are not better because of the diffraction limit.
I did not get to this video as a way to "bash" the A7r2, which is obviously a very nice camera. I'm not mentioning the "L" word here at all. What got me here was another discussion about whether lens performance in general can vary at an aperture like f/11 or F/16.
In trying to understand this issue I came across articles which claimed the diffraction limit was f/13 on a 12mp FF but f/11 on a 24mp FF camera. Several of these stated outright, that unless you planned on shooting fast all the time, a larger pixel was more desirable because diffraction did not set in so soon and thus you could use f/11 and f/16 to full effect when you wanted DOF.
Now some think when you downsize a 42mp to 24mp the diffraction effects, which are greater at f/11, will magically disappear and the two images will be the same.
I don't really know, but I would like to find out, because I do shoot alot of landscapes and I do use f/11 often
I can't say I don't find it interesting the inventor of CMOS basically says outright: the MP race is a marketing scam. I'm not qualified to say to what degree it's true or not, but again I'd like to understand the issues better.