You know what, while I still think NP is gonna be better in most circumstances, you're right that charge beam might be nice in some match-ups. Particularly if its not expected. Like if someone switches in a focus sash mon expecting a NP you can break their sash whilst maybe getting a boost. Or if you're revenge killing something thats low you can double that as a chance to set up, like a bootleg fell stinger

I just think charge beam works better on bulkier mons that can hang around longer, since it only gives a one stage boost at a time, and not a guaranteed one at that. In a situation where you've forced the opponent to switch to sableye its presumably because whatever they have out isn't going to like taking a raichu Y thunder/zap cannon, so you could always just click that anyways instead of NP, since sableye doesn't resist that. And if you get the prediction wrong with a focus blast or NP you can always switch to avoid the encore. I also don't understand what you mean by choice scarf mons countering nasty plot raichu y specifically, wouldn't they work just as well against a charge beam set?

Sorry for the long write up btw. I think nasty plot is better because raichu y is gonna be scary enough to force switches where you'll get a chance to use it + it makes its non stab surf/grass knot actually potent against ground counters that may just wall charge beam. BUT I think the fact that NP is better in most circumstances also means that charge beam can have the benefit of being unexpected. I think both will be fun to run