I was going to post a rant on a different thread, but I decided to open a new one since it really was unrelated. Also, there was an old thread on this topic, but it contained a lot of no longer relevant information due to its age. This thread is also directed at newer members who may have been told by some wise mentor that they don't need more than 6mp.
.. so onto the rant.
The more megapixels I have, the better. I'll take as many as you can give me *up to* the resolving power of the very best lens that is theoretically possible, and that's why I always just want to choke chickens any time anyone says "but you don't neeeeeeeeeeeed that many megapixels." This tiresome argument is still repeated ad nauseam is predicated on the assumption that what your camera spits out is perfectly framed, that all you care about is some final printed image, and that you're not going to print billboards. All of these arguments are just wrongheaded.
Main point: megapixels + ultrasharp lens = crop for days = better final image.
The truth is, *most* of the times, having a lot of MP and an ultrasharp lens has the potential to make your final images a lot better. Consider that the Canon 5Ds resolution is 8688 x 5792. Now, assuming that my lens resolves that kind of resolution, I could chop away a full 2/3 of that image and STILL have more than enough for most clients and large enough to send to most magazines for printing. Think about it: 66% of what you shot gone, and yet the final image is perfect.
The creative possibilities this opens up are tremendous, and to me it's all about creativity and all about the final image. I absolute DO NOT CARE about what my photo looks like coming out of the camera as long as it gets me to where I want to go. Indeed, I often shoot "wrong" because I know that will take me to my happy place in post. This includes not just framing, but also exposure.
An example that I will reiterate: I used to shoot beauty (basically super closeup shots of the face, or parts of the face, showing off makeup) with a 105/2.8 micro lens at about ƒ11-ƒ16 because getting in super close means you're killing your depth of field. A few weeks ago, though, I decided to shoot beauty with my 85, stand back, shoot ƒ6.3, and just chop the hell out of the files. And yet, I was still able to give files that were twice as big as what the client required. Bonus, I could make the picture tall, wide, square, whatever—I wasn't constrained by the original composition. It just didn't matter because the files were just that robust.
Thus, another bonus of megapixels is that you can save a bunch of money on lenses because the robustness of your files allows you to achieve equivalent final images with fewer lenses.
Imagine if you had a 60mp camera that shot 14fps. If you had an ultrasharp long zoom lens, you could shoot that bird as widely as you wanted, or that quarterback as widely as you wanted, make sure you had as much of the scene as you wanted, hack and slash in post, and still have great images. Heck, maybe you see something in post that's off to the side that you suddenly realize is super cool. Win!
As a corollary, pairing an unsharp lens with a high megapixel camera truly is just a waste. I have a good friend who owns (and loves) the 58mm/1.4g from Nikon. It produces *very* pretty pictures, but is not anywhere near sharp wide open. I used one for a couple of different photo shoots when it just came out and I thought that there was some kind of problem with the focus fine tuning because when I looked at the photos, they weren't at all sharp, and in post I couldn't crop much without ending up with a blurry image. Sure the photos were pretty, but I was trapped to nearly my original composition. Add to this the fact that if you're shooting at ƒ1.4, you need to catch an eye in focus, and to catch an eye in focus, sometimes the highly centralized focus points require you to compose for a crop in post. It's a catch-22. Point is, if you're shooting an unsharp lens, you might as well use a Dƒ because high resolution sensors really *are* a waste.
If you have a high megapixel camera, you *need* ultrasharp lenses to take advantage of them.
My goal right now is to end up with a final image of at least 5120 × 2880, because that's the resolution of a 5k monitor, but yeah, that's a bit overboard. I'm more than happy with 3840 x 2160, which is 4k, though. With a D810, I can almost about chop a file in half and still have a 4k image, but most clients are just fine with 3000 on the long end, and that's all I usually submit for magazine publication
Megapixels are a game changer, it's just many people are still thinking about them in the wrong way.