It's irritating to have it as a choice on Special:Preferences for new users, tricking them into believing that this is a useful alternative, and well-supported option.
Another insight into unmet user expectations, from T181770#3806461
Excuse me, on the Appearance tab, there are 6 skins to chose from. Nowhere does it say that any of the skins are less supported. The idea that only one of those skins is "the supported interface" is improper. All 6 skins must fully supported.