When to build QSS on non-ADC?

I just saw Poppy with a QSS in one of the CLG vs SKT games and that reminded me sometimes in my own games I want to buy QSS on a non ADC/AD champ but I'm not sure if it makes sense to do so. If you could, please break down your answer by role i.e. it never makes sense to do so as x role but sometimes it's good for y role.

As mid I'm under the impression that you would just take cleanse instead of buying QSS, but I usually take ignite/exh so I forget that cleanse is an option. I know Zhonya's is an option but you can't use it if you get hard CC'd and it seems like often Zhonya's backfires and lets the enemy team collapse on you.
As support I usually play Zilean and I would want a QSS in games where I get chain CC'd and die before I can even ult myself or anyone else e.g. vs a fed Kennen.

Would it make sense for a Wukong/Kennen so that they don't get cc'd during their ult and allow the enemy team to escape?

I'm guessing that the answer might be "you need to position better so that you don't need a QSS" and that QSS is an inefficient item on non ADs since they don't benefit from upgrading to Mercurial? But at the same time, if getting CCd makes you totally unable to do your job in a team fight I would think it's worth it to get the QSS. Would Banshee's be a good alternative, especially later in the game when you want the extra defensive stats and have more gold?

Thanks