It's not for cooling as the 'turtle shells' that the pumps sit in are completely uncovered by the time the inb'd mains are down to about 28500kgs.
More than likely it is to ensure that that maximum amount of fuel is available for use from the outbd mains as the fuel pick ups are just outbd of the (inbd) tank wall, and space is at a bit of a premium. Not that you'd see it (hopefully!) but the aft boost pumps will run dry first.