Bitcoin-QT could send the "change" back to the originating wallet by default
Be careful about your terminology. Bitcoin-QT already does this.
It seems to me that you meant to say:
Bitcoin-QT could send the "change" back to the
originating address by default.
But what should it do if there are multiple inputs from multiple addresses for the transaction. Which one should get the "change"? Furthermore, re-using addresses decreases security since the public key becomes public knowledge when you spend an output associated with a particular address. Re-using addresses also reduces anonymity.
As an alternative, users would be presented with the two options
If this is what a user wants, I thought there were already other clients that allow this.