Good point. But actually these situations are pretty rare. Why should you split up your party in SP if you're not forced to do so? And in these rare cases another companion could be allowed to speak on your behalf (quest or situation related).
Sometimes splitting up is not voluntary, like that dungeon example. Who knows what else like that is later in the game. Making exceptions for these situations is not in vein of systemic approach Larian has in DOS.
Also, I take it both of your suggestions in the first post assume that dialogues with npcs take place only once and if botched no second chances? This will just encourage save scumming, which is bad enough as it is in DOS. I'd rather have another go at an npc with another companion.
