Okay, so you're saying that you use her ult to pick people off? Then you're still stuck with her glitchy, unreliable passive proc for the bulk of the fighting. But she has so much invested into her ult - its long duration and she gets a massive 80% AS boost, which makes it appealing to use in an all-in situation. But, she also loses the burst damage since she doesn't get her passive anymore.
It just sends mixed signals. Do I want to stay with Quinn and have to put up with her terrible AA animations and buggy passive, but keep the range? Do I Ult and risk going into melee range? Without any good escape, 0 CC, 0 survivability/defenses, that's a risky proposition. So I'm using it to pick them off? So do I just ult and then recast it to get the execution damage? Well, then you just wasted all of the potential use of the ult just for the damage. If she needed an execute that should be the central focus of her ult, not the Tag Team stuff.
Yeah, its good for chasing people down, but if you're chasing people down at low health then you're already ahead, and if you're actually winning with Quinn (given all of her other problems) then you're just being handed an easy game to begin with.