Why should I care about static characters? Honest question.
But the main characters are not the only ones. The standard format of episodic television like TOS, and of similarly episodic book series like Trek novels used to be, is to tell stories featuring guest characters who can grow and change, with the main characters as catalysts and participants in their stories, or to have the guest characters be people the main characters have developing relationships with over the course of the story (albeit reset at the end).
Again, though, I don't see the value in arguing over different individual tastes. You don't have to like every single book. Not every book should be written exclusively for one person's tastes, because other people with different tastes deserve to have books written for them as well.