User blog comment:TheTobarMethod/Do AP's Need A QSS Upgrade?/@comment-1153246-20130907055406

Except people don't usually upgrade QSS until they have to, to be honest. QSS isn't there to rush into a Mercurial, Mercurial was introduced to give bruisers a way to build QSS and still be able to end with lategame damage instead of being forced to keep an item that offers them less stats than they need, but is still required for whatever forced them to buy it in the first place. QSS is ALREADY strongly situational, in most situations tenacity/proper positioning will get you by just fine. Zhonya's also exists for a very good reason, it wasn't recommended item on non-AoE by accident.

I think we also have a differing definition of APC, given that I can't think of many AP mids who have no real utility in the first place. Some, like Ahri and Fizz, have less than say Lux, but things like Ahri charm and Fizz ult are still pretty heavy displacement, both in lane and teamfights. Karthus is an example of an APC missing real utility, but he's more of a special case anyway because of what his AoE does in teamfights even after he's dead/used Zhonya's. The only APC I can think of with any total lack of good utility is MAYBE Malzahar, simply because it's a channeled suppress and thus locks him down too, but AP QSS would make him even LESS popular than he already is.

TL;DR Zhonya's is fine to use, BV if necessary (it should still be up once per teamfight unless you make a point of taking champion damage between them), but a QSS upgrade for AP is likely just going to either be completely unused similar to Moonflair's original stint on Rift or, and I'm just spitballing here, you can STILL BUILD QSS. Just don't upgrade it, build your other items instead, lord knows an APC needs that more than MR or Health/CDR.