What the two weapon limit did in the original Halo was to increase the complexity of encounters by making you pick up the weapons that you felt were optimal for each given area. Sometimes switching weapons meant that an area would be easier, but the following one would become tougher. Thus, weapon, and ammo management became important. I think this gave the game a great reason to try the campaign multiple times - to see how things would play with different weapon combos. Two Betrayals is a the best example of this, as are the areas where you can't pick up any human weapons.

Halo 4 has the two weapon limit, but pretty much eliminates any of the benefits it provides by making a lot of the weapons work the same. In Halo, running out of pistol ammo meant you either had to find more (encouraging exploration), or switch to something else (encouraging planning and conservation). But, when you run out of BR or DMR ammo, and a lightrifle or carbine drop everywhere, then there's not much to think about is there? Just pick one up and keep going on about how you did before. You are neither encouraged to explore or conserve, because more can be easily found.

I think for the future, 343 should eliminate weapon duplication, or else allow an entire arsenal to be carried like in 90s FPS games.