According to a report from @dchan at the 2016 All Hands, zh.wikipedia.org readers in Hong Kong commonly see articles in the main language code, i.e. zh, rather than in a specific variant. This means that the wiki will be a mix of Simplified and Traditional scripts, sometimes changing from one to the other in the course of an article. This makes the content very hard to read.
In practice, most readers did not discover the variant drop down, and instead carried on reading, with difficulty.
This is presumably because they use a browser which does not give any valid variant in the Accept-Language header. LanguageConverter::getPreferredVariant() will then fall back to $wgDefaultLanguageVariant, which is false on all WMF wikis, and then to the main language code.
I suggest adding geolocation, and then finally picking a winner arbitrarily (say, Simplified). Content should not be served from /wiki/, redirection should always be done to a specific variant URL.
Also, we should improve the visibility of the variant selector, say with a GuidedTour-style callout on first view.