From: Iowan in MD/DC
One argument in favor of using TRACOM (as kind of an Uber Reserve). I always keep large quantities of obsolete aircraft in stateside squadrons as training squadrons . And it's a good use for HQ squadrons that are too small to be any use in combat.
B-18's,A-20's,A-29's,SNJ's and any obsolete fighters. And of course SOC's upgrade to Naval Attack squadrons, so they are useful too. Basically you never have enough 2nd line aircraft for training.
I'm not sure how well this works for the Allies, but for the IJN training program in my AI games I like to spend a day each on vastly enlarging some carrier-capable units by transferring all a/c off of Hiyo and/or Junyo when they arrive, transferring a carrier capable training unit onto the CV, and resizing to 53 planes. Granted, if the unit was previously restricted it is no longer restricted, but I'm only using it for training in the HI anyway.
If that can be done with any of the Allied squadrons, particularly the USMC ones, it would help... Though not with the 4E "problem". As Japan at least, I have no problem building enough aircraft to fill out the training units. Or replacing enough second line aircraft through upgrades.