I do think it's a problem that non illithid parties get no mechanical benefit at all. I'm glad that those of you who enjoyed playing no mechanical benefit at all did so but I think it's problematic that is
I never played MotB but I played the BG series and in it resisting the pull of evil came with mechanical benefits - lesser benefits to be sure but benefits nonetheless. It does sound like the MotB system is superior to the one we have now.
Now it is true that non tadpole parties get some minor alterations in the dialogue but those a simply a mild flavoring of the text. You can either get something like "you haven't been using the powers from the tadpole" or "you've been using powers of the tadpole, good"
Nothing about your relationship changes the game acts as if you trust The Guardian and will be working with them. You don't even get an opportunity to state the obvious: you are asking me to empower the tadpole while I am seeking to have it removed, that puts us at odds.
This becomes more problematic later in the story when it's crystal clear that your guardian is an evil figure who is manipulating you using morally objectionable means but the game assumes that they only reason Tav's objects is that Tav fears the other. Tav is disgusted by the alien physiognomy and not the morals of a brain eating, enslaving, gaslighting, manipulating crime boss. When we do express our upset The Guardian always has the final word - even on no tadpole runs.
Tav: "I'm nothing like you" Guardian: "You are more like me than you wish to admit and you would do well"
Oh. Guess that statement stands then. I guess I my options are to be annoyed that the NPC gets better lines than my character or not . . .