For starters, I have no idea if attacking the Soviet Union is brillant or the stupidest idea to ever hit WITP, but I've always wanted to try it. Seydlitz's AAR helped convince me that it can be done, and I think that by waiting until just after Phase I operations are complete, I can use some of the Southern Army forces in the attack (while not having to worry about having trouble securing the Southern Resource area).
My long term goal is not to crush the Soviets (which would probably be impossible), but rather simply to sieze the trans-Amur region and the Soviet ports, destroy their airforce, fleet, and pocketed armies, and establish a defensive line in Manchuria (see map).
My reasons are as follows, in increasing order of importance:
1. The Soviet Union has resources and oil located close to Japan.
2. Soviet equipment is realtively bad until 1944-45. For example, they only have BT tanks (no T-34s until 1944 or so), and a 1942 Russian infantry squad is worse than a Japanese one.
3. Soviet aircraft replacement rate is low to non-existent, and their early planes are short legged and weakly armed.
4. I believe my opponent is relatively cautious, and something like this might just force him into taking risks with his carriers that he might not othewise take (he might try to invade somewhere if he thinks I'm distracted).
5. I believe I have a window to strike a blow against them in 1942 before the western allies are prepared to counter-attack. I think this game might go the distance, and if I wait until 1945, the Soviets will come in when I am least prepared to deal with them. Vladivostock will be a dagger aimed at Japan and I will have no free troops to stem the tide of brand new Soviet equipment. In 1942, I can use air, land, and naval forces, that will not be available later on. These extra forces, in concert with the Kwangtung army, might just be enough to remove this long-term threat. Meanwhile, the KB still rules the waves, so they should be able to safeguard the empire while much of the Southern Army is tied up in the Soviet Union.
6. I feel as though I am helping out my pal Adolf, and doing something that although it would probably not have won the war historically, probably offered the Axis powers' best chance (in my opinion).