I'm no expert on the subject, but that never stops me from giving an opinion:
Let's say you wanted to a closeup picture of a circuit board. 21MP would allow the viewer to zoom into that picture and see very small traces This is sometimes useful . . . if you need to take pictures of circuit boards.
As far as I know, as long as it's a 35mm SLR, in that 8+MP range, the real big deal is what lenses it will accept. That's where the "magic" is. Oh, and that's where the money goes.