Montgomery's %90 sucessful remark is of course misleading. Number of objectives captured becomes pointless if you fail to achieve or strategic goal (that was to gain a bridgehead on Rhine , advance north to isolate Netherlands and get Ruhr. Market Garden wasted a good deal of elite airborne troops for that goal) Still those %90 objectives captured was not strategically or operationally complately worthless. Yes 21st Army Group front extended but new reinforcements were coming from US everyweek and British and Canadians would have to enter Netherlands no matter what to secure Scheldt anyway. Airborne corridor up to Waal provided a shield to any German counter attack towards Scheldt and Antwerp. Nijmegen airhead was also useful to launch Operation Veritable later to outflank West Wall. In fact maybe it can be claimed that Market Garden was strategically more useful to Allies than fabled German airborne invasion of Crete in 1941 (Operation Merkur) In Merkur German paratroopers secured Crete with heavy casaulties but Crete at the end proved little strategic value except to provide airbridge supplies to Rommel and his Panzer Army briefly in 1942. Neither RAF nor USAAF did not have precision accurate and constant raiding capability from Crete to Ploesti as Hitler feared. So Crete was worthless to Germans essentially and they tied huge garrsion to there. Market Garden's secondary gains in September 1944 were more tangible in short term although operation was itself failed strategically and operationally
Another point that has been overlooked in this thread is that, once on the ground, the American Glider Pilots were non combatant and had to be protected. How many members of the Airborne troops landing with them would have been required for that purpose. On the other hand the British Glider Pilots were trained to fight as infantry and it is recorded how they fought with valour and distinction at Arnhem. Perhaps, because of the distance that they were being dropped in advance of the Allied front line, this might have been a factor leading to the British being selected for the task of taking Arnhem.