When braking lightly in fast, sweeping corners in the wet, i.e. near the limit of traction, you want about the same braking at both wheels to avoid slipping. Since the rear brake is already weaker for the reason Brucey states, I wouldn’t want to add to that inequality with a single-pivot brake at the rear only.
I think single-pivot brakes have useful advantages, especially for light male riders who don’t need much mechanical advantage, but I’d want both brakes to be the same.