I happen to own a AR-10 without the forward assist. Much of my shooting is done in very very gritty desert conditions on a large dry lake. The fine grit there gets into everything and is a pain. One failure I've had several times if a failure of the bolt to go into battery. Because of the design of the AR the gun can not be disassembled when it fails in this way. The bolt if short of full battery by as little as 1/3 of an inch or so prevents the upper and lower from swinging apart when the takedown pin is pushed free.
To disassemble it I have to take a small allen key, put it into one of the holes in side of the bolt that is visible through the ejection port and pound the bolt forward.
The forward assist is a much faster and simpler way to accomplish this. I think the Army did well to insist on it and I would be reluctant to buy another AR pattern weapon that lacked it.
I agree with you there... I've got an old Colt CAR-15, and it doesn't have a forward assist. I've never had a problem with the bolt not going into battery, but then I don't fire it that much. Being left handed, wer glasses, and tall, there's not enough stock pull, but worst of all, there's no shell deflector "nub" on the upper, so every ejected shell hits my glasses! I've also got an AR15-A2, and we get along much better.