One reason for wanting to start with as many pixels as possible is that every time we manipulate the image, we lose a little quality - resize up or down, get rid of barrel or pin cushion distortion, blend images for depth of field, or stitching, or correct perspective distortion and each of those steps degrades the image a little. This all on top of the original raw file conversion and any manipulation the camera does to raw images - raw isn't always as virginal as one might think as the manufacturer does some noise reduction or some sharpening before you get to play with it.
All these changes do add up. If I sharpen an image, I'm playing with the data and losing just a tad of resolution to gain in sharpness. Run my image through Akvis Enhancer or any other programme that adds local contrast or helps shadows and highlights and even more loss.
Dodging and Burning further damage the image - which is why this is the last thing I do. Even all those fancy blending layers and adjustments distort the data even if they don't destroy it.
And all this doesn't even consider cropping.
Personally, I'll be happy to be have a camera with 36 or more megapixels.