Oh it was definitely rare that an interjection would actually change the outcome of the dialogues, but that color and flavor you mentioned does a lot to make characters feel more alive. Same goes for the way party members just randomly start conversations when walking around in BG2.

Worth noting I always play with the Gibberlings Fixpack installed. IIRC the vanilla game is really sensitive about party members having to be physically close to the talking NPC to interject into dialogues.