I have transferred a lot of pokemon from the legacy games to Pokémon X. The set includes nearly all pokémon from Kanto, Johto, Hoenn, Sinnoh and Unova.
The XY pokédex clearly distinguishes which pokémon I've caught are from Kalos (with a pentagonal symbol) and the ones that are not (with a pokéball symbol). This makes around half of my Kalos dex with pokémon not originated from Kalos.
I plan to complete the national dex in XY too.
Now, is there a necessity to have all of Kalos dex filled with Kalos-origin pokémon? Or is it just an additional functionality (like showing if you have English/Japanese/German/etc. version of the pokémon)?