And which is more fun? Say you were the US, you get the m9, p90, m60, m40,m16, m4, basically all of the best weapons. Russians you'd get an AK-47, AK 74, RPD, rpg-7, and the dragonov. Who cares about unique weapons per side when you can use whatever weapon.
Unless the guns were all the same basically like battlefield 1943, that would be dumb. Weapons that look different but shoot the same are boring. Then with a massively popular game like cod4 people would just complain. Say one side is more OP than the other. Which it would've been, a team using m16's would likely beat a team using AK-47's.
Times are changing, no one wants "like good old times" we want new and better. To me, fixed classes is an old idea that has no place in fast paced first person shooters.
Also, you play as special forces units which have a far greater range and flexibility of weapons than normal units. They aren't just picking from the m249, m16, and the m4, they can use whatever they can get their hands on.