The problem isnt really her role, its her kit.
Even rito admitted, that quinn was quite a failure. If you go through every ability youll see why shes ( in my opinion) garbage.
For example: take her passive;
first: its random, for all we know, nobody can predict randomness. So nobody can really take full use of that passive; ye I know you can activate it with your E and its pretty handy in a 1vs1, but that doesnt help in teamfights to shred the tanks.
second: it gives your attack speed when you got a point in your W and valor gains the attackspeed all the time; the problem with that is, that NOBODY upgrades his W first.
You know, thats still an acceptable/meh passive, since its annoying as heck IF you get marked.
Q and E are ok-ish as well, a blind is good and a somewhat escape/knock up ability isnt really bad either.
What really bothers me, is quinns ult.
1.) If you are an adc, you need to kite or get out, since you are squishy af. Well ok if some1 got you, you fly away. Ok seems fair.
But thats it. You wont be able to kite or do any dmg whatsoever. Unless you find a target to assassinate and thats my second point.
2.) Unless you are in 1vs1, you get instantly killed if you use your..... AoE-Execute xD. This is hilarious and sad at the same time. But mostly sad. If you look at the numbers, youll see that this %%%% deals almost no dmg. Missing %-health. This is just bad design. Graves is laughing at this. Even without his godamn cigar.
And since you didnt upgrade your W to rank 5, you got the lowest attackspeed on valor. So you are a suicide bird.
Final result: Quinn is an adc, that wants to poke like a teemo, assassinate like zed and kite like a vayne. But she isnt anything like those champs. She can trade pretty well in a 2vs2 or 1vs1. But she has one of the %%%%tiest designs.
Dont get me wrong. If you want, you can main quinn. You can play her and have fun, nothings wrong with that. You can even get fed, win and clmib the ladder, since nobody plays her.
But there are reasons why you dont see quinns that often: this champ has a bad kit nonetheless.
Overall, have fun and enjoy the game. These are just my reasons why I think rito fcked quinn up.
If you think I made a mistake, correct me, Im always open for a new perspective. :)