Well definitely it have zero benefits to play alone. Well it whould be if every additional party member whould decrease the XP gain with 50% until we get just 25% of the original XP. I mean even original BG had this.
But i dont want this because it whould effect the full game. An we whould see agan this game journalist crying about the game. And the next day we will see patches like this.Increased players' HP bonus in Explorer Mode from +50% to +100%. Again.
I think it's much more better not to touch the existing difficulty. Just add another tier. And then every one will be happy "mybe".