When you can only consider that the best sensor of Nikon ( D4 ) has 16 Mpixel, and on D 800 has 36 Mpixel. The number of Mp tell you only how large you can print it. 3 Mp = 5×7″ ; 6 Mp = 13×19″ ; 10-12 Mp = 24 x 36 Mp. Also, if there are to many Mp, you will have a bigger amplification ( the pixel to small = an electrical signal to small ) , and for sure more noise on the photos. Today, the number of Mp is more a marketing and sales argument as something helpful. I have Nikon D 7000 – 16.2 MP, D 700 – 12.1 MP.
Also to consider having more MP is not just the larger you can print, but also how much you can crop without losing to much details. A D3 or a D700 only had 12MP and you could enlarge that quite a bit too. It is possible to print out pictures of 10MP of a D80 on a size of more than a meter.
It’s true, not always you need that big MP power. But sometimes, you want to crop the picture you took. Having so much detail available, you still can do some “post” cropping.