I'm glad they did, since not everyone had Ranger.Weird why they let that thing to be able to be traded on the GTS back then. That was how I got mine in Platinum. I registered it thanks to a book I found in the Pokémon Mansion.
Doubt Pokémon Ranger will be a thing in a while so they could as well find other ways to introduce the Manaphy Egg. I wouldn't mind an in-game event where the player gets to save a Manaphy Egg from poachers in-game and gets it as a result.
Maybe and I hope they do.