I'm not trying to taboo everything of the form "X is Y".

Consider an analogy to the argumentum ad hominem fallacy. I think it's correct to dub this a fallacy and say it's not a legitimate move in argument. However, some people are stupid, some people are evil, and it may be correct and proper to mention that they are stupid and evil. It just can't be doing the heavy lifting in an argument. Certainly calling people stupid and evil is useful as a slogan, it's useful for introducing evidence against them, it's even valid in some kinds of arguments (For example, "Bob is stupid, so we probably don't want to let him design the nuclear plant.")

I think Worst Argument in the World is the same way. There are some legitimate uses for statements of the "X is in category Y!" form, but actually doing the heavy lifting in a philosophical argument is not one of them. I'd be pretty happy if people just stopped doing it entirely, but I admit that it's possible (although I think unlikely) to keep using it and always be responsible with it.