The air defense may have been superfluous, but the training wasn't. It was also useful to have safe areas to move combat veterans to in order to give them someplace safe to rest, recover, and not lose their minds. One of the effects on the bombing campaign over Germany was to prevent that.
The colonies also brings up a long-term strategic error of the imperial administration: actively promoting local industry and education would vastly expand the pool of manpower available to the RN and RAF.
The Commonwealth didn't have the resources to send much more to Malaya. I suspect one of the resource problems was leadership — the British generals there don't seem to have been from their "A" team — and training for the ground forces. The IJA and IJN were planning on attacking a well-trained, competently-led force. The British didn't seem to be planning on defending against one: Japanese forces seem to have been consistently underrated until met in combat.