Why can't you see decals (bullet holes, blood, scorching) on scenery, forge objects or players? Is it the same reason that tank shells and bullet casings fall though such geometry?
Your Xbox is not the same as your PC. Your PC has much more memory, probably a better graphics card, etc. to be able to show these "decals". Most console games don't have them, and for a very simple reason, they are very costly (in memory) to load. Forge World is actually a great example of this. Stand on some side of Forge World and look towards the other. Can you see any details? This was shown to us early that the Halo Reach engine loads more polygons the closer you are in a player's sight. No one is looking for bullet holes beside them after they get shot at, so why include it? It's a pointless waste in memory that can be used towards better things.
Considering that Halo games in particular are used for farm more than just killing each other there is a reason to have it. And in fact Halo used to have it and may still in campaign. I think the reason that bullet affects and stuff do not show up on forge objects and what not now is because it's too hard to track what surface of that forge object got hit. Back in goldeneye it worked to have the same bullet hole show up weather it was on glass, stone, metal whatever but now if people see a bullet hole in glass they don't expect it to look like metal. So rather than track each part of a forge object or a spartan body they just do without them