It's not about the lines, but the dialogue system. It's not made for dialogues with more than two participants. And of course every such dialogue would require extensive scripting in order to let the companions participate in it. So of course that's a different take on the topic with a lot of additional work just for the SP part. I heavily doubt Larian would do that.
One of the very first conversations in the game, where Dallis is confronting Atusa, has at least four participants. My memory of D:OS's toolset is very fuzzy, but if simply changing the speaker requires "extensive scripting," then they're in trouble.
