The three images below, in no particular order, are made from a 6 megapixel Nikon D40, a 12 megapixel Nikon D90, and a 24 megapixel Nikon D7100. The same lens, lighting and compositing technique was used in each image.
Below is how most everybody today views images; starting off as a thumbnail, then perhaps enlarging via a light-box feature to take a closer look. Examine the three images, and ask yourself if you feel any of them stand out as "better" or higher quality. Then, you'll have your answer as to whether or not megapixels matter.