It depends on the angle we'll approach the subject. Let's imagine for a second that they will work just as any cRPG companions with their own will and thought process and personnality. It's only when you start thinking about how all the things they say may actually be deconstructed to work in MP that they'll loose value. Maximilian the redhead warrior dwarf who despises Elfs because he is a Dwarf may very well have a different reaction in MP because the player will not choose [race] as a reaction but [redhead] or [warrior].
Well, that's why I think that SP has to work differently in DOS 2 than MP. If Larian wants to maintain them in one thing SP is - and I hate to say that - already doomed and only an inferior (emergency) version of MP for people without real friends.
I get it that it's complicated for Larian. They want this game to be even more co-op centered than DOS which will make the SP mode naturally worse by literally erasing all traditional premade companions. I mean, they enhance on the character traits you chould choose from in DOS for your second char (like loyal, bold, aggressive,...) by fleshing that out in (hopefully) well written origin stories. But that literally erase every traditional companions from DOS 2 while you still had two of them in DOS (though already way less impactful and interactive than those in old Bioware games).
I don't know but I have the slight feeling that Swen and Larian don't get what party CRPGs are all about for many people in SP. It's not just "playing PnP with yourself", actually not at all. Playing a game like BG2 solo was no decision based on lacking the friends but a conscient decision to have a narrative and emotional experience that is only mechanically(!) based on PnP, but not narratively. The origin of emotions in PnP MP is completely different to the origin of emotions in SP CRPGs. And this has a HUGE effect on game design of course. Let's take (narrative) choice & consequence, the bare bone of every good RPG. This works quite differently in MP and SP. As I've said before, in MP a decision or choice is much more about the interaction with your friends. "What does the group want and what's my stance in that?" Hence, choice situations are designed according to that process of player interaction (humor for example often works much better here than serious topics). In SP, on the opposite, narrative choice has a quite different range of options because such choices don't depend on others but only on you. That's why SP RPGs are often "serious", offering you difficult moral choice situations that challenge your ethics, mind and emotions on a whole different level than it would while playing with real people, having a real life chat all the time. A lot of that has of course to do with immersion and player psychology stuff. But there are numerous reasons why SP RPGs work differently than PnP sessions or MP/co-op experiences.
Making the SP just a copy of the MP (with "intelligent" bots) is imo the completely wrong way to make a good SP RPG. I mean, there is a good reason why so many people complained about the narrative in DOS. It was not because the writing was all bad but mostly because the whole game was designed for MP from the start, with SP being an afterthought that was 95% based on the MP experience. And it shows, because the narrative offers too little meaningful content for SP-only people. And I'm very doubtful that Larian actually got the problem "right". Increasing the writing staff doesn't solve the problem if the design for SP stays the same - or gets even worse, with now all four companions being "intelligent bots", originally created to be taken over by real people in MP. I mean, that goes well beyond just companion design, but about the whole narrative. I know that it's hard for Larian but they should maybe stop telling people that this game is made for SP as well - because it isn't, at least not primarily - and they only create a lot of expectations in players, especially those who don't know every detail of the envisioned design (and maybe even get fooled when Swen tells them that the narrative will be so much better this time by having a bigger writing team...). But sure, given the fact that >80% of their customers only played the first DOS in SP there is a significant dissonance between what Larian envisions for DOS 2 and what the vast majority of their fans want...
As I said I sadly didn't really play a game in which they really mattered, so I'm biased toward thinking they are mostly useless and irrelevant - or at best likable, but still pretty random. I want them to tie with the story, with the ongoing events, to matter. I maintain that DA:O had characters better tied up to the story vs, say, Mass Effect 2 ( especially 2 ).
So either matter, either serve a purpose, or don't stay in my way asking me to retake your castle when I'm trying to save my friend's life ( hi, Nalia! )
But you're still in that black or white mode, either "irrelevant" or "meaningful". There is a whole range between. And of course companions are useful in a traditional party RPG because they help you in each and every fight, even without having a personal quest tied to it. That's why they are actually called companion instead of just NPC. So whatever you say, they are never useless for gameplay, and consequently, for the narrative. You can even see it like that: Nalia joined your little party and helped you saving your sister's life (which she does in each and every enemy encounter you face!) not only because she was bored to death. She hoped that a capable group of adventures could be able to help her in return as well. One hand washes the other. You help her reclaiming her castle and in return, she stays with you until the very end, risking her life for your ass numerous times. Isn't that a small price to take? I mean, it's kind of weird that you claim that companions should be believable persons tied to the overall narrative and when they have actual goals you criticize their whole design just because you can't leave your selfish view?
And then again, companions are much, much more than just their personal quest and I don't know why you try to reduce them to that. They give context to each and every decision you make. But of course we might even see narrative itself in a whole different picture. For me, good stories are not so much about the plot, but primarily about well written characters and their relationships and interactions with both other characters and the world. That's the core of every good story (no matter the medium, works the same way for novels or movies) and that's why well written companions can enhance a narrative so much. Interesting characters have their own meaning and value just by being interesting. They don't have to enhance the plot for having value, not at all.