Yes, I agree with all of you that the helicopter around World War II was at its infancy. Still, what I am saying is that it is plausible that a better type of helicopter could've been developed had more resources been allocated for its development.
The helicopter wouldn't be suitable for an air-to-air mode, of course, but as pointed out in one of the replies here, it could conceivably contribute to the battlefield as a transport or close support role. I doubt, though, that a helicopter could've helped the paratroopers at Arnhem because that is generally a built up area. Lots of hidden places there to shoot down a chopper.
There are, I think, many WWII campaigns whose results could've been altered had this aircraft type been available. One such campaign could be North Africa.