Using guidance by itself is not an exploit; it's baked in to the rules of 5e. If you know a check is about to mb made, it's ~expected that you'll cast guidance. The only somewhat exploitative part is that, by casting guidance, you're clearly casting a spell on yourself which NPCs should react to. However, most of them probably won't know what spell you cast; all they'll know is that you cast something.
BG3 would be more interesting if spellcaster NPCs recognized guidance cast during a conversation and reacted to it. Maybe they have a similar reaction as you casting Charm Person on them - loss in approval and/or becoming hostile.
Maybe non-spellcaster NPCs could accusatorily ask you what spell you just cast, and you'd have pass a deception or persuasion check that it was nothing important or else they'd become more hostile to you.
However, implementing the above could be too much work for too little gain. At the very least, it'd be sensible and a simple fix to prevent guidance from being cast while talking to party companions (e.g., SH giving guidance for Tav to persuade her of something).
