2 games ago, I put up all 6 in the SZ 6 battle, 4 on a BB and 2 on a DD. I destroyed both, tipping the odds in my favor.
The anti-bombardment is best used to defend other targets like islands from the unaccompanied bombardments.
There are plenty of times that all 6 are insufficient to change the game, so 1 probably will not either, but if you save 1, it can flub the bombardments in second and subsequent wave attacks. If Japan is in this position, its probably losing. Its better to use them to win (deter or change odds at a critical moment).
It is not true that 1-2 fighters do not change the game for SZ 6/Japan on the defense or offense. There are many times later in the game when everything has another job, all the air is dead or occupied on the Mainland/Island defense and without a few fighters, you can’t project any attack/defense to dissuade anything. You have to have a DD but having as many 4-air as possible opens new lanes of attack/shuttle bombing optys.