I agree with the OP too. It's like pretty much every achievement-featured game on steam, x-box, Google play etc out there: an achievement for completing every mission, and one more for going back and making sure you did everything perfectly.
Even if it didn't yield much, it'd still feel great to earn and it'd give players (ok, the ones who play more for fun, not the hardcore ones who wouldn't bother) something to work on?