They will never admit it until it's done, but here's a poorly kept secret: even a good portion of the "4 is fine" crowd once actually offered the option in the game would gladly bring along five companions, not three.
It makes for more interesting party compositions, more diversified battle strategies, gives to the AI more targets diminishing the focus on your "squishy characters" (or giving you more options to actively protect them) and gives you more occasions to inject the action queue between enemies moves; it gives you significantly more chances to use more types of valuable equipment, it allows you to progress concurrently on more companions quests and hear more banters and interactions between characters at any given time.
What's the downside, again? Oh right... It may upset the current shitty balance, for encounters that need to be retuned anyway. And it MAY force the devs' hand into actually making controls out of combat bearable. Oh, the horror.