wasn't one of the reason to make event pokemon has egg move was to save players breeding time, might as well cut grinding time too.
wild pokemon are sometimes encountered in their non evolved form even when they're already past their evolution level, spheal in ruby is an example, so it doesn't break the game logic. Why not make rare candies sellable btw, a person who is lazy like me can just grind easily and those people who enjoys bonding with their pokemon can train them normally. Of course this is for the later parts of the game, where the level discrepancy between the event pokemon and the wild pokemon is too huge. Grinding to 40 50 is kinda ok, pass that its too much of a pain. The beauty of selling rare candies compared to rebattling trainer is that people have to manage their resources instead of getting money and levelling at the same time