Given that you can switch around dockyards between projects without any penalty, why not produce convoys then you need them. Sure later game ships are better then early ones but reaching the end game is not granted and you could be better of having a stronger navy in the early and mid game then a strong late game one.
As Japan you may want to use all your dockyards to build warships.
I'm not saying you should wait until 1940 to start warship production and sit on enough convoys to last to 1948. I'm saying that whenever you're in a "gap" where the newest warship you can produce is not really that modern or at least won't be modern by the time you go to war, that's probably a good time to stock up on some convoys so you don't need to use your dockyards for that in 9 months when a new and much stronger ship class is unlocked.
Convoys in stockpile is a redundancy, I agree with that: Whenever you have exactly as many convoys as you need and no extras, you have spent the maximum available amount of IC into power for that moment in time. Your suggestion to produce convoys only when needed would thus be the most efficient if all ships were of equal quality and if you need your navy to be at maximum possible strength at all times.
However, the navy needs only be at maximum strength when you need it to, which is usually periodic peaks of time starting in 1940 or so. As long as you don't have redundant convoys during these peaks you will have the same amount of IC spent on combat power for these periods as if you stocpiled earlier, and any previous stockpiling is not a hindrance. As a player, you can often know when these peaks of demand for naval power will be. As Britain for example, you will experience such a peak of demand for warships when first entering war with the Axis (particularly Italy) early in the war, and another when the Japanese join the party. Each of these peaks can last for a week or a year, but they are temporary: One side will gain the upper hand and can rest parts of his fleet, the other side will be forced to port for a period. You as the player can plan for when these peaks will come and a competent player will aim to have the maximum strength available at those times. One aspect of maximizing strength for those is to not have convoys stockpiled at the time, as these represent IC that could have been spent on warships.
Another and equally important aspect is to use the available IC for the highest quality ships possible. This is why stockpiling convoys can actually be efficient: Producing 100% convoys in the first year and building 100% warships in the last 3 yields the same number of warships and convoys after 4 years as producing 25% convoys and 75% warships over 4 years, but the one who does the early stockpiling will have better warships and older convoys at the end of those 4 years.
If you as Japan "use all your dockyards to build warships" as you suggest, then I wish you good luck in using those warships for anything useful. What's the point of conquering an overseas empire if you don't have convoys to return the resources and supply your troops abroad?
