Ny reason to actually hurry

The only method to change weapons on the Xbox 360 version is using the radial dial, by holding the LB and using the right-stick to select your weapon.

My opinion, is that this can be a really fast method of selecting the weapon you want. Granted, the game goes through an animation of switching out the weapon, but as long as you know what you want, you can quickly hit both and have a weapon switched out in no time at all. Hardly what I would call cumbersome, but just my opinion.

To my knowledge, there are no particular benefits to dual-wielding the same weapon. The level 6 unlock serves pretty much the same purpose as any other unlock: To keep you from doing what you might otherwise do so that you have something to aim for. Cynical way of looking at "progression," but it works. Likewise there are no "benefits" to unlocked clothing items that would necessarily justify their not being available initially.

That said, I do tend to dual-wield the same weapon more often than not, because 1) As you said, you level a single weapon up faster instead of spreading it between two, which is nice just because you'll get the good stuff (lightweight, barrel upgrade, etc) faster. More importantly... 2) Two weapons that behave the same way can be easier to use than two that are slightly different. The obvious case is differing ammo capacity (try using 1911 and PT92 without extended magazine on the 1911), but even vanilla 1911 + Bull can be frustrating when one weapon's firing rate is different than the other. In order to maximize both, you'd have to have a very strange clicking pattern. Or in other cases, the damage dropoff may be worse on one gun versus the other.

So yeah, no strong benefit, just potentially easier to use, and another thing to unlock.