When we get the same souls over and over it should power up the one we already have instead of forcing us to waste time selling it and risk accidentally selling the wrong one, etc. Furthermore, several people have said they quit playing after they made such a mistake.
Such a system is faster for players (no screen change) and reduces load on servers (power up a dupe vs deliver a dupe, and then process change to my team and process a sale).
It would also increase the variety of play because long time players will eventually end up with other characters being powered up as well as their mains so they would be more likely to use them.