I like fighters a lot, and will buy them with all nations, although I’ve yet to do so with Russia. With Russia I think it may make more sense to just focus on land units.
But with the other 4 nations they can serve a dual land/sea purpose that other units can’t. They are cheaper than DD’s and just as effective in attack, while 2 tanks can’t threaten any ships.
Also their range is great, esp if you have carriers in certain spots, for example a ftr that is placed in Sz 60 can make it to Sz 34 in one turn, then to Europe to help defend Germany. Or for the US sz 55 to sz 12 in one shot.
Again with the exception of Russia, I think the other nations should always try to have at least 4-6 ftrs minimum on the board. With the US I’ll hit double digits in ftrs easy, UK maybe 6-8 although they take the brunt when sinking Germany ships so I usually end up with about 4-6. With Germany, I’d like to get 7-8 ftrs depending on if Ukr was attacked Rus 1, but 5 and 1 bom is certainly good enough if you want to concentrate on land only. And Japan is sort of a wild card, they start with 6, so pending on how Germany is doing I’ll mainly stay between 6-10. If Germany is in real trouble and I have Japan earning over 40, I’ll try and go 1 ftr per turn, so I can keep a steady stream of J ftrs to Europe if the ftrs are needed there.