In 1941, the Sub is 100% useless. It can not back up UK’s Fleet, because by the time it comes to Russia’s Turn, Germany has already went and destroyed UK’s fleet (g1). When it gets to UK’s turn, they will sink the remaining German subs/fleet and build their own fleet including a Carrier. With a carrier and other ships in their fleet, the Germans will never be able to challenge the UK fleet, AND if they do, it will be with Planes, and so the Russian sub can not be taken as a casualty and so it is 100% useless.
In 1942, the Sub is Actually useful. Because Russia goes first, Russia can move their sub to Seazone 2. This causes Germany, if they want to attack sz2, to use 2 sub, 1 ftr, and 1 bomber. The Russian sub can be taken as a casualty, letting the UK BB take one more hit before being destroyed. Because of the Russian sub, the Germans should end up with just their ftr and bomber left, instead of also a sub too. Big difference? No, it is not. Especially because german subs defend on 1 and will be destroyed the next turn whether there are 2 or zero anyway.
In short, I think that after Larry Harris changed the rules so that Planes can not hit subs (a good rule change imo), he should have replaced the Russian sub with a Destroyer. (remember that the OLD submarines attack and defend at 2 / 2, and the new destroyers are also 2 /2. I feel that the reason the russian’s had a sub in the previous AA incarnations, was because destroyers back then were 3 / 3, and so it was a little too powerful to give them a dd)