In that case they are +4 IPC for Caucasus that turn. However, they are not in an tactical advantage and Russia has suffered no loss for loosing it.
Net: Russia Tactical Advantage with larger then normal stack in W. Russ. IPC Gain +2 (WR)
Net: Germany at no advantage, IPC gain of 4 in Cauc, IPC loss of 2 WR, net 2 IPC gain, tactical disadvantage due to larger then normal stack in WRuss.
The way I see it, Germany gains +4 IPC from the territory, and +1 IPC from the 1/3 chance of killing a unit with minimum value 3 IPC (one infantry), so net +5 IPC for investment of +3 IPC. Germany also benefits from tying down the UK bomber for that turn, which is not trivial; a UK bomber can do a lot of damage to valuable naval units.
The Allies gain a tactical advantage from killing a forward placed German infantry, as well as the fact that the UK, not Russia, depleted Germany’s forces, which in turn allows Russia’s forces to mass and therefore be more efficient.
Russia gains nothing by holding West Russia because I believe Russia should hold West Russia until Germany has heavily massed at Ukraine. That is, Russia should probably be holding West Russia anyways.