Well there are only eight companions in this game, 5 with the PC and only 3 on the bench. It would be nice if you could switch them in and out without the benched characters falling behind, get to know them all, and use different ones for different situations.
Of course we don't yet know if they will have a bench system like in NWN2 and Dragon Age, or if switching requires officially kicking someone out like in the Baldur's Gate series. Also not sure how the adventurer hall will work into this.
I suppose the adventurer hall could charge you money to make a character above level one, higher levels costing more.