it does provide a certain advantage to the player that has unlocked the more correct weapon for a situation.
Nothing is stopping you or anyone else, to unlock the same weapons.
Personally I prefer to unlock what I want in any order as my playing style will differ greatly from another player.
Then unlock what you want, when you want, by completing the appropriate challenges.
Appearently weapons would be through leveling, while attachments and clothes would be from challenges
You appear to be wrong. - in the GamingBolt vid, Stern says you unlock weapons
and attachments in Challenges. - http://www.youtube.com/watch?feature=player_embedded&v=pd_RO9aj2y0
It has been known for awhile, that clothing is obtained simply by leveling and playing the game.