I'm not saying that every city was fought over historically, but they were all of strategic importance. Are you suggesting that Leningrad, Smolensk, Vyazma, Moscow, Stalingrad, Rostov, Kiev, Kharkov, Tula, and so many others were not hugely important objectives? The reasons varied - production, political value, rail and road junction, winter shelter, etc, but the primary geographic objectives of almost every major operation included the capture or defence of centres of population, production, transportation or supply.
Now, in the game, what must the Soviet player defend, apart from the Urals? He has no reason to defend ANY city in the game.
You can't just take a single city as an example to support your point. If you insist on doing that, then I can take Moscow, Stalingrad and Leningrad (also quoting John Erickson) and beat you 3/1 :)
The player may lack the capabilities to defend, say Kharkov, but he shouldn't lack the desire.