Initally (and actually for sometime aferwards), there was resistance to the repeating rifle. Because it would encourage troops to "waste" ammunition. And ammunition was expensive. Look how long the US Army held on to the Trapdoor Springfield, even though there were better (more combat effective - greater firepower) arms available. Also, the Springfield was cheap (the design came from an Army officer, so the Army essentially got the gun for free).
There was a lot of resistance to the M1 Garand prior to, and well after its adoption, primarily for the same reasons.
It took the cataclysm of WWII to thaw the attitude of ordnance officers making them more receptive to the new. The pendulum has swung the other way, currently, and the system is much more willing to at least test out new designs and concepts.
Is full auto necessary? clearly not. Just look at all the history of war prior to full auto weapons, The victors won without full auto, and the losers didn't lose for lack of full auto fire.
Necessary? No. But it is effective, and a great force multiplier, which overall means the cost to our side in blood to accomplish the mission if lessened with the availabilty of full auto fire. Therefore, it is a desirable thing. Very desirable, in many circumstances, and since it helps us, and we can, why not have it?
All else being equal (and it almost never is) bigger bullets tend to work better.