It's pretty easy to see that they are thinking quite realistically here. It appears that they are wanting it to appear like the real world military, where soldiers can opt for what gear they want, or how the mission suggests certain equipment. I mean, do US Army soldiers walk around in their helmets every hour of the day? Do they have full body armor every hour of the day? Does every soldier carry around the same exact gun? If you looked at any given set of photos of troops from today, you will see a different setup each time.
I mean, you see it in-universe too. Check out Halo 3, for example. You see Marines with helmets with goggles, helmets without goggles, no helmets, backpacks, no backpacks, and so on and so forth. Not to mention the entire arsenal at their disposal. In ODST you have troopers outfitted with different gear, add-on gear, etc.
Is it so out-of-the-question to think that maybe these guys got a little stuffy in their gear, and that they didn't necessarily need it to stand around and chit-chat in a field that's not a battle zone? They have a truck nearby, so you can make the assumption that the rest of their gear is in the truck. As for the Assault Rifles, maybe that's what their unit was issued, or that their mission required heavier fire power.