I guess it could help to have forces in Spain (providing they aren’t wiped out) if you want to conduct a dual assualt on the next turn; an amphibious assault AND an attack from Spain. This being the strategy if you don’t have enough transports, or you don’t want to build more.
And having a “Washington Express” going on sure would help matters. But, again, if I were Germany, I would take Spain and Portugal right back from you ASAP. I wouldn’t let you hold them. The problem would be the constant threat of re-invasion from the US and Britain. I think it would come to the point where Germany really cannot turn back the combined Allied armada, at which point it will become a land war where it may be anyone’s battle.
I guess we will have to see how it works out.