Battleships useless?

[GAGA Extrem]

The problem with building carriers as Germany is the amount of tech and research you have to sink into it to field anything worth the effort. It's a lot easier tech-wise to focus on battleships and either light cruisers or destroyers. Also remember that battleships are cheaper IC wise, when you factor in the aircraft you have to build for carriers.

I don't think that a CV fleet is really that dependend on techs.

As GER, you can research CVL and skip the CV techs until you have won your war in the east and attack the USA.
All you need is to keep your CAG techs up to date and provide some air cover. That doesn't cost too much leadership - and a fleet consisting of 6-12 CVL can easily dominate the RN, provide decent support everywhere around Europe and even threaten the Atlantic.

Although that is a SP tactic. In MP games, the struggle for air superiority is usually quite intense, so CV can become mauled pretty easily. In that case having a backup fleet relying on "tradtitional" firepower can be an immense benefit.
That reminds me, btw, of a game where two of my soviet submarine flottilas wrecked havoc among german supply lines and forced them to withdraw most of their air units from the front.

So unit efficiency is really a question of "how do you play? How does your enemy play?"
Hehehehehe...