I was reading this interview with some CoD4 guys over at 1UP, when I found the following part:

GFW: You can carry only two weapons at once?

VZ: Yeah, you get one primary weapon — an assault rifle, submachine gun, sniper rifle, whatever — and a sidearm. Attachments work with pistols, too. Then there’s the Dual Wield perk, which lets you swap out the pistol for a second primary weapon. You sacrifice Stopping Power or Juggernaut, but you’re packing two weapons.

You need to use a perk to be able to have something besides a pistol as your second weapon. It’s quite common right now for me (and many many others) to have a rifle as your main gun, and then pick up a shotgun as a secondary, close-quarters gun. Or a machine gun and shotgun. Or whatever. Now to get this previous free feature, now I’ll have to pay with a perk slot. I don’t like this at all.

Call of Duty 4: Modern Warfare PC Preview