Will F:
...The HP doesn't pump a great deal more than the engine requires, and the spill is a fairly closely calculated flow that is more a safety consideration than a potential Gross Tankage circulator.
Not quite true, except at SL. But you gotta consider the whole envelope.
Think about this: The HP pump flow rate is directly proportional to core RPM, and is independent of altitude. Thus if there's a small flow margin at SL, there's a big excess at altitude.
Further: If idle speed is 70% (for discussion purposes...), then the HP flow rate at idle is 70%, where the actual engine burn rate is maybe 10% (or less). Therefore the extra 60% has gotta be bypassed.
STILL further: Idle at top of descent combines the altitude bypass AND the idle bypass, meaning there's a potential local overheating (through recirculation).
It's this significant excess, or bypass, fuel flow that is the question. If it's not gainfully employed, you get hot spots & cold spots in the fuel system.