Language is organic. It's not that somebody somewhere chose to overcomplicate things. It's just that if you want to be an international Spanish speaker, you need to be aware that you might hear it either way.

I've always said 'cupiera,' but I've probably heard 'cupiese' more.

It's hard to wrap your mind around the subjunctive with a purely academic approach. But in spoken Spanish it's used a lot. You start to pick it up quickly.

'Cuando puedas' is ubiquitous as a polite way to say 'when you can / at your convenience.' 'If / then' statements are another big one. 'I'd have (conditional) all the money inthe world, if I was (subjunctive - fuera / fuese) a wealthy girl.'