Quite simply, because you have to go through the entire Act 1 Battlefield without any teleporter stones, and if you try to scarf-and-sell as much loot as I did, that last dungeon is quite a distance away when you're encumbered.
It also removes the problem where you have dropped one 'stone in the main game and/or one in the 'fields, and then transfer between acts.
On your other points, there's no question that the Battlefields are incredibly generic, but they're a bolt-on and not a truly strong standalone game feature.
I'm sure Larian could do a much better job with their implementation if they were designed into the game engine from scratch.
-- Retro
