I'm gonna give you the correct explanation, When CS came it was one of the first FPS games requiring skills and ofc all geeks wants to show off so you started doing noscoped, and when we all tried noscoping on COD we noticed that it doesnt take any skill at all, just luck and something as near noscope as possible is ofc if you scope as little as possible, Thats how quickscope was kinda brought to COD. But its not about that its a good tactic, its all about to show off, just like 360's and stuff but 360 is too hard for noobs to do so they are more rare, (and lets face it, not many good FPS gamers play MW2, lol)
Quick scoping does have a tactical position in mw2...at least for me. I often run intervention with any quick swap weapon so close range if I miss a qs I can still take the enemy down. But at mid range using a d-eagle or pistol is not nearly as easy as just quick scoping the opponent. But tbh there are alot of noob quick scopers around and unless your going to get good with it then there is no tactical position of qs'ing. But anyway if you can combine a mix of hardscoping, quickscoping and using secondary when you for example or jumping everywhere and obv can't hit a halfscope midair lol then you would have a pretty strong tactical advantage over your enemy.