The Bell 47 is far too fragile. Machine gun fire would damage it, and helicopters are somewhat fragile to begin with.
The helicopter seen in the opening scene of MASH is a relatively simple design and was first flown in March 1946, what if it had been in March 1944 instead? What kind of effect would this have on the war in Europe? In the Pacific? Perhaps even on the Eastern Front?
One of the helicopter's best uses could have been simply to scare the crap out of the Japanese. From firebombing to innovative carrier setups to the A-Bomb, our military innovations in the Pacific served 2 purposes:
1) improve military performance
2) propaganda to convince the Japs that no matter what they do, we will always innovate faster than they will.