To determine to hold Leningrad at all cost is very tempting for many players ( the more confident the player is, the more tempting). but defending Leningrad required a big commitment and once it failed, it often results losing many troops. Also, Germany could just ignore Lennigrad ( I did it in all my games as Germany).ORIGINAL: timmyab
The other thing to think about is the early release of 4th pz group and some of the infantry for use elsewhere. The key to a 'Leningrad light' defense is keeping these units in the North for as long as possible. Deception can play a part here.
On the whole I think it's correct to defend the South aggressively against a strong Southern opening under the current patch. You not only gain time to evacuate the industry but also you can move more of it because there are less combat units to rail.
Brian G's defending in Lenningrad is quite smart, he evacuate it early and make sure Germany need the port to kill it.
That way, Germany need more time to get it. It is always good to think before what will happen, so you see things just work as you plan (your opponent didn't foresee it).
I myself always want to fight hard at Lennigrad as soviet, but that requires 1:1=2:1. It is fun, but too risky, even with 1:1=2:1. The tempting is that if Soviet fend off the Germany in Lenningrad, it won a early game. But if the Germany knows the business, Soviet should not do this.