ORIGINAL: christian brown
What the F are you guys talking about?
Where is the disadvantage?
As noted by GKar, the German troops will be more vulnerable to a counterattack in Fall if France has been substantially reinforced. The Allied troops may have increased in quality by that point. The Germans might not be at 6 evasion yet. If they aren't, that will hurt in case of Allied attack in Fall and Germany's own attack in Winter. Plus the Allies will get the winter bonus if attacked by Germany in winter - making their fort, in particular, very tough and even West France somewhat defensible. Maybe I'm missing something -- but I don't think an attempt to take France in Winter is a guaranteed success in every case and there are substantial potential costs to trying it (albeit substantial benefits as you noted). It will probably cost more in manpower, which could affect German's plans.
Anyway, I don't disagree it can be a good strategy. And if it always works for you, keep doing it. I just wanted to challenge your assumption everybody does it! [;)] One or two of the still few people I have played with - who I think are good players - didn't do it. It could be coming in every game, but we'll see.
EDIT: There is also the factor of Soviet war readiness - if France falls in Winter, this (plus the other war readiness increases so far) can be enough for the Soviets to reach the 2x production multiplier a turn earlier than they otherwise would if France falls in Spring.