The forward assist has a use on my hunting rifles. Numerous times, I've needed to load w/o making the noise created by letting the bolt slam forward. Easing it closed and bumping the FA is much quieter than the rack & slam method. When hunting in snow conditions, I've fallen or had snow laden branches dump snow onto my AR. With the dust cover closed, the chance of snow clogging the action is lessened. With a snow ball packed around the bolt it might work or maybe not but either way it's likely a cleaning will be needed later. But with the cover closed, I'm more confident the rifle will function and I'm not going to have to warm, dry, and clean the rifle after a long day in the snow.
I accomplish what you describe without the aid of a forward assist. DPMS BCGs have a oval divet in the right side which acts as a forward assist. you just stick your thumb in there and slide it the rest of the way into battery. I'm not sure if all BCGs have this as I am too lazy to go check my other AR but I would be willing to say that I could do the same with my 9mm AR which does have a forward assist and dust cover. as for snow falling on my gun... I just had the great fortune of going on my first week long hunting expedition out in the wilderness. there was much snow in the first half and the second was spent getting pooped on by the trees as the temperatures rose again. one thing I learned after the umpteenth time of getting creamed in the face with an inadvertent snowball when the guy walking in front forgot there was someone behind him before letting that tree branch swing back behind him....your body heat keeps your gun warm enough that snow in your gun is a non issue. however no matter what the conditions when you come home from hunting you should always set your gun near the fire to dry out properly, not doing so is just asking for rust.