When someone completes the pokedex I believe they should get a small reward, such as if you complete the Seen Pokedex you should get a Epic Egg. If you complete the Catch Pokedex you should get something like a egg charm which either gives you better luck when opening eggs such as the shiny egg, it should decrease the chance of getting bad pokemon (Rhydon, Golem, etc) or it could shorten the time spent on breeding a new pokemon, I mean lets face it, not many people would like to wait 36 hours just to get their Eevee.