A lot of my favorite weapons are early unlocks... P226, m416, ak5c. The only part that would suck to me personally is having to do assignments or the campaign to unlock the weapons. I hate how they did that with the campaign.
But when it's all said and done I personally like to earn my shit but everyone is different. But what makes a good player is not the weapon there holding. Anyone can hold an ace 23 doesn't mean there good.
But I do see the other side how it pisses people off. You dump hours into the game and they get there shit in a click of a button.
But like Nesty said they might have everything unlocked on another console or they don't have the time to put the grind fest to unlock there weapon of choice because there busy with life and just want to hop on and have everything at there disposal.
I have played this game so god damn much now I don't even notice people who are using a weapon that is not unlocked to their level yet.
I just shoot'em and move on to the next one.