There are several good reasons to get an ac instead of a battleship. The first is that Germany already has fighters to land on the AC. So with the AC (plus fighters) you get a defense punch of 10, with a count of 4, compared to the bb’s punch of 4 and count of 2. The second is that you won’t get many bombards out of that battleship unless you are using it in the Baltic. You might get the one bombard when you attack England, but ultimately that one bombard shot is pretty useless compared to 2 fighters (also 20 ipc) that would participate in every round of the battle.
Another reason is acs extend the range of your fighters. Unlike ships, fighters can’t be blocked.
Whether you intend to go for London or go for Moscow, there are better ways to spend that 20 ipcs.
Agreed, all good points.