If landing at MLW according to flight plan due to tankering, what happens if ATC gives direct tracking? This can sometimes be significant if expecting long STAR or SID, or perhaps you even get radar tracking to a relatively short final for a runway close to inbound track. If this happens close to destination there might not be the opportunity to increase fuel burn. I remember one time getting direct tracking after take-off at Casablanca to right base at Paris Orly! In such a case it was unnecessary to adjust fuel burn for max landing weight. Other past last minute track shortening was at Kai Tak when arriving from the north. Significant track shortening close to destination however, leaves less opportunity to adjust.
Other considerations are that there might be a possibility of last minute pax or freight additions. It would also be wise to allow for enough landing weight margin to enable the operator to fill the empty seats just before departure. A reasonable margin beats having to de-fuel or refuse extra pax! The same consideration is needed for the next day departure. Note that de-fuelling requires a tanker, probably empty, to collect unwanted fuel, and in my experience there could be a very long delay for this, resulting in loss of departure sequence or slot time that could impact the operation of the aircraft for the rest of the day.
Some captain adjustments to tankering fuel might appear without good reason but years of experience have influenced the decision.
The scenario is interesting and many good points have been raised.
It is noteworthy that some CX ops devote considerable resources to tankering fuel selection.