It's not about the lines, but the dialogue system. It's not made for dialogues with more than two participants.
Why would we need a dialogue system here with more than two participants? I haven't seen companions interjecting if one of them is already talking to someone, nor should they - it's a mechanic that can be used to do some sneaky business while npc is distracted. And if they all have something to say game shows their lines one by one. So I don't see what's the problem.
