User blog comment:Callofduty4/Modern Warfare 3 Weapon Proficiency and Equipment trailer/@comment-27.32.58.102-20111102110412/@comment-3323759-20111103000041

I was under the impression that the proficiencies were only active when your primary is equipped, but we'll have to wait and see. Either way, the knifing proficiency may be extremely annoying, depending on lunge.