Imagine a scenario: you(with an smg) and a bunch of other guys from you squad are sitting inside a building, you spot enemy squad at the other end of the street, 300m from your position. In real life, you would silently alert your squad, call one of the guys which has a rifle to slowly crawl to your windows so they don't spot your cover, you show the enemy to other riflemen so they could open fire, each taking a separate target, etc. etc. It would be an example of good teamwork.
The problem with the games is that it happens 10 times faster and you don't have all the options the real world offers. In the game the enemy doesn't move in one formation, they are scattered around an arena-like map so they could be even behind you, and you won't have the possibility to effectively use everything your "multitasked" squad can do. At least that's what happens in 90% of the games I've played. (dunno, maybe you played on some super mega organized server where people play as a super mega organized squad, I didn't see many of those)
To sum this up - imagine the real world war where you play as a lone wolf, cause that's what 90% of the game plays like. And tell me now - won't u use all the available means to survive?
I want to stress once again - for a realistic implementation - a penalty system must be added for those who carry 2 guns. This will improve the game for lone-wolves, and wont change it for the ultra organized squads.