An interesting derivative of a Russian held Korea is that it denies the US player a Korean IC.
As Japan, simply letting the Russians hold Korea indefinitely would essentially eliminate the possibility of a US IC on mainland Asia (assuming that US strategy is still viable in Global).
The Japanese player can always choose to attack the Russians in Korea to reduce whatever stack is there - and/or - keep a sub or two in SZ6 to negate Russian income - and/or - launch their own attacks into Siberia to force the Russians to reduce their Korean garrison themselves.
The IPC loss to Japan (of 3) could easily be gained from other conquests, including Russia’s own original territories.
Sure, its kind of “gamey” to leave Korea (particularly if its empty) in Russian hands, but ultimately could be to their benefit if it deprives the US of an IC build later.
The obvious wrinkle here is if Russia is cleaning up in Europe, and therefore can spare 30 IPCS to build their own factory there. But any Japanese player should be able to see that move coming rounds in advance, and simply move to take Korea pre-emptively.