Anyways, you've a maximum of two companions already.
Some folks not on the pc might like to have 2
human companions instead of one human and the mutt/bot. I don't understand why they chose not to allow that. Well, I do, perhaps (2 humans are more effective than human/mutt) but at least for me, Arcade/Boone or Raul/Cass or whatever other combo is MUCH more entertaining than Rex/Arcade.
But the hardware limitation aspect is a valid point, especially regarding consoles, and I think it is likely the reason it was restricted to two. I don't think I'd want more than 3 as a default option, because of the overcrowding/follow distance issues that crop up, like mentioned.