In real life, doors have to open for people to go through them without serious injury. Adding sliding doors to transport choppers is a good idea, not only because it adds a nice visual touch to the aircraft, but also because it could increase chopper capacity (troops sitting on the edge of the door, possibly being able to fire out). The leaning out the door bit isn't super realistic (for me) because I sure as hell wouldn't be dangling my feet out of a chopper 1km up, but it will look good, add a little firepower to unarmed aircraft and also it's not too far fetched, we've all seen photo's of troops doing the same thing in real life.