Noise isn't information. If you take an 8MP image, and upscale it, then add fake noise, you'll find at some point you've matched the (noisy) high ISO image of the same scene shot at 24MP, because the noise in the 24MP image was just that... noise. Not actually information from the scene. Or conversely, if you take a noisy ISO 6400 24MP image and start scaling it down, you'll reach a point where the noise becomes imperceptible (it averages out). This is why you never see noisy photos on instagram... The images there are only displayed at like 3MP.
People fall into this trap every time a new, higher megapixel image sensor comes out. They look at a 16MP ISO 6400 image at 1:1 and compare it to the 24MP ISO 6400 image at 1:1. That's not a valid comparison. If you downscale the 24MP image to 16MP, then maybe you can compare. But chances are nobody but the photographer will ever see the image at 1:1 anyway, they'll see some 3-6MP downscaled version on the web.
The moral of the story is: if you want to crop a lot or make a giant print (i.e. benefit from that 24 megapixels) then you're going to have to shoot at ISO 200. If you accept that you will only make smaller prints or only display on the web, then you can crank the ISO as high as you want (the higher you go, the smaller the dispaly MP you can support without the image appearing noisy).
