If the other guys would not leave the keys in the ignition the GI's would not steal them. A GI who does not have a vehicle and is walking will when the opportunity presents itself take anything, even LandRovers, and use them for transportation and besides if you wreck it or blow the engine up no one really cares.
I know that I would not trade a M1 for an Iraqi T-72/62/55 any day, unless it was made of gold.
When the M-16 was first used in combat it was not really ready for use unike the AK-47 that had it's bugs worked out already. I used a M-60 on a gun truck for five months before I got shipped back to the States in 1973.