What happens if that Italian fleet is destroyed before a Japanese CV arrives? No way to use those ground units in Africa now where Italy needs to be making money and just as importantly, costing the UK IPCs. Your stuck with 2 infantry and an artillery a turn to reinforce france with or more likely attempt to hold Italy with since there is no fleet to cover you against a landing.
Playing as the Allies, I would take full advantage of a weak Italian fleet as soon as possible.
An Italian fleet is a great investment. Fleets are better on the defensive in general with the exception of subs, so getting a CV and extra fighter into SZ14 is a top priority for Italy with me.
How are you going to sink the allied fleet by turn 3? If it is on t1 it is before Italy can do anything anyway so that is a moot point. If it is T2 then Britain bought only airforce which would mean that is one less turn Germany has to worry about the Brits and can focus more on Russia. America only has 2 bombers so that’s not going to do anything. If it is T3 is still has to be sunk by Britain as the Jap carrier would be there by America’s turn. Not only that you can probably move the navy to the black sea or something to keep them out of harms way for 1 turn. There is no way Britain can sink the Italian navy with ships and have a sustainable invasion force to drop in Europe. You are putting youself out of place for no good reason and giving Germany an easier time to kill Russia. The more the Allies screw around in the Med/Africa the more time the Axis has to kill Russia.
Plus if Italy builds a fleet the Allies can just choose to ignore it. Maybe send in an American suicide Tranny or 2 to get units in Africa. If they did then Italy just wasted money for no good reason. I could however see the point of a second transport (maybe even a third) so one can double bombard the cauc to oblivion. Other than that I would only build navel units on special occasion.
And fleets are not better on defense than Infantry. 1 Infantry unit costs 3 ans defends at 2, 1 destroyer costs 8 and defends at 2.