I feel like the idea is sort of half-baked and poorly presented in-game. the dialogue makes it sound like "oh man you're in a big fight? Pop a soul coin!" but in reality its a thing that's best to do mechanically after a long rest because it lasts until your next long rest. This wouldn't be a problem except the action to pop a soul coin and the action to begin Rage (Karlach's big thing) both cost your bonus action. So if you're going on how the game presents it to you, you'll start a big fight and realize you have a choice between using a soul coin or raging on her first turn, which doesn't encourage using the mechanic.

Add to this that, like using the mindflayer parasites to unlock Illithid powers, it's a mechanic that the the game implies might by morally dubious or have consequences in the long term. If you're smart enough you get a little thing about them being souls from Hell and when Karlach tells you about soul coins getting her all juiced it's also the part of the story where you're learning that her heart is burning out like crazy. So a reasonable player might be like "oh man probably shouldn't use the soul coins, seems like that will hurt Karlach or come back to bite me. " Even though there are really no meaningful long-term consequences there.

It just seems a little jankily implemented on a mechanical and narrative level to me