I suspect the 17,000 lbs minimum comes from a series of ADs issued to address various fuel pump failure modes and the potential for the pump to create an ignition source under dry running conditions. Those ADs required the pilots to monitor the fuel quantity and shut the center wing tank pumps off at a specified CWT quantity (7,000 lbs in climb or 3,000 lbs in cruise) before their inlets had the potential to be uncovered. The ADs required that, if you had any mission fuel loaded in the CWT, there must be a minimum of 17,000 lbs in that tank. That 17,000 lbs minimum was based on the normal fuel burn sequence using CWT fuel first, and was intended to ensure that the tank would not hit the 7,000 lb climb shutoff level during the high workload initial climb portion of the flight when the crew was unlikely to be able to effectively monitor the CWT level and shut the pumps off on time.
Here's an educated guess about the other aspects of the MMEL item: The MMEL entry appears to be trying to account for at least two scenarios with that limitation. The concern is that the single remaining main pump in the tank with the inoperative pump may also fail. 1) If that occurs early in the flight, it may necessitate a turnback or diversion due to trapped fuel in the affected tank (and in the opposite tank due to the imbalance limit), and therefore insufficient mission fuel. You may therefore be landing with nearly all of that center tank fuel, so they require you to treat it as part of the MZFW so you stay within landing weight limits if that scenario occurs. 2) If the second boost pump in the affected tank fails late in the mission when you are on tank to engine feed with equal amounts in the four mains, causing you to lose access to the fuel in the affected tank, they must have calculated you needed some contingency fuel in the center tank for that scenario, and it must have been less than 17,000 lbs. I highly suspect the 17,000 lb minimum would have been driven by the fuel pump ignition source issue I described above, because it would be an unusual coincidence for the contingency fuel to be the same 17,000 lbs amount.