I would argue that the Philippines invasion was entirely unnecessary.
The U.S. certainly didn’t need it as a base of operations, as Nimitz had already effectively bypassed the Philippines by taking the Marianas.
US air superiority, as evidenced by the Great Marianas Turkey Shoot, was well established. The threat of Japanese naval and air assets stationed out of the Philippines was minimal. The US had also had no need to send significant assets within striking distance of the Philippines. Support of resistance there was maintained with submarine and covert resupply.
The US submarine campaign had also hamstrung the IJN, leaving it desperately short of fuel. Invading the Philippines nearly played into Japan’s hands by placing a large transport fleet within striking distance of their base on Borneo.
It is a certainty that any attempt by Japan to engage in a Mahanian style fleet engagement on the open ocean, either in a second sortie against the Marianas, or against an invasion of Iwo Jima, was doomed to fail.
Moreover, the US had been effectively supporting Philippine resistance against the Japanese, to the point that many smaller islands were literally free of Japanese forces, and many Japanese garrisons were surrounded by territory under the control of Philippine forces.
While many Filipinos were supportive of MacArthur and his efforts to retake the Philippines, it is questionable that bypassing them would have had any negative impact on the resistance efforts there.
The invasion of the Philippines also resulted in the Battle of Manila, which destroyed the city and caused hundreds of thousands of civilian casualties.
An objective look at MacArthur’s campaign in New Guinea, against forces that were largely ill-supplied and un-supported by air, naval, or artillery forces, should have indicated that another long campaign in mountainous jungle was counterproductive…it would do nothing but engage forces that had already been written off by Japan, and did nothing to further the goal of actually forcing a Japanese surrender.
One could argue that capturing the Philippines made the job of our submarine crews easier, but this is a false assertion. US subs never operated out of the Philippines, and the Japanese never stationed any assets there that might have made their job harder.
About the only real reason for invading the Philippines, aside from fulfilling MacArthur’s promise, was to prevent the possibility of reprisals against the population by Japanese forces as the war wound down.
Considering the casualties that occurred anyways as a result of the invasion, I believe the possibility of such reprisals was an acceptable risk.