Quickscoping, at it's core, is just an exploit of the auto-aim mechanic. A glitch, if you will.
Don't get me wrong, it looks pretty cool, and if you can do it, it's actually kind of fun. But here's the real problem...
First, quickscopers are hated by damn near everyone. It's often viewed as nothing more than over-glorified cheating by the majority of players.
Second, I used to play quite a bit, and I've seen quickscopers go both positive, and negative. But typically, they go negative, and I'll explain why. Your connection speed determines if you can be a functional quickscoper. Now, that is not the end-all be-all; I've personally gone quickscoping with three bars on a consistent basis, and done just fine. But that is rare. Before anyone starts throwing around comments about me being bad, I came from PC, where quickscoping was a pain due to a mouse. Timing was everything. My friends all consider me to be an average/slightly-above-average quickscoper. So skill is not the issue. Bottom line, if you do not have one of the fastest connections in the lobby, player hitboxes will likely never be where you think they are. Unfortunately for the quickscoper, the auto-aim mechanic will always pick what the quickscoper sees, as opposed to what the server sees.
Third, quickscopers consistently lose matches for their team. Before I become a pariah, and people start chanting OpTic Pred's name to the heavens, let me remind you of my second point. Connection speed is over half the battle. If that quickscoper does not have a good connection, he's dead. I did the math back in MW3, on a three bar connection, based on number of encounters (seeing an enemy and taking a shot) vs. the number of encounters I actually lived through. Something like 13.8% of encounters, I lived. Those are not good odds. Granted there are exceptions to everything, but the vast majority of the community will experience the same problems. So, let's say you survive a flat 10% of your encounters in TDM. If you have a total of say... twenty encounters in a match, which is about average for the average player in the average match, then you will survive, theoretically, two encounters. This means you scored two points for your team, but gave the enemy team eighteen points. That boils down to a .11 K/D ratio, which, is about the only place K/D matters to people. The odds are NOT in your favor.
Last, I'm all for playing the game how YOU want to play it. That being said, I prefer to take the traditional sniper approach. Yes, I mean pitching a tent. I have no shame in doing what I can to ensure my team wins, because where K/D means nothing to most people, W/L is what's important. High K/D and high W/L are almost mutually exclusive these days. Again, exceptions and all that jazz. But, if you want to quickscope, be my guest. Just don't harass me if I snipe you from a distance, in waiting, while you run around getting owned by Peacekeepers and R870's.