Wow, that is some hardcore drop rate. It's not even that good of a weapon unless there's some really annoying light-based enemy later in the game. Well, I don't play Pokemon games, but I know I will have to put in some effort if I want all the shinies. :D
It is actually just a weapon that is really good if you get lucky early game, since Zeke and Bernard can use Scythes it really helps hitting 2x per attack! I honestly thought it was a bit more common than 1/250 I might have to lower it to be honest haha. Later on in the game through the main quests you can get a better one which is one of those guaranteed drops you were talking about earlier! :)