Right, that's why it was initially useful. Now that lots of people use the question, though, even mediocre developers will make sure to come to the interview with a list of criticisms of their favorite language.
Re:Why Ruby is not my favorite language
by jamesbritt on Wednesday December 10, 2008 @11:41PM
(#393966)
"... even mediocre developers will make sure to come to the interview with a list of criticisms of their favorite language."
Which is fine if, when pressed, they can back up what they say. The question (like most good interview questions) is largely an excuse to help guide a conversation and some exploration to avoid canned responses.
I used to ask interviewees to discuss what they didn't like about their favorite language. If they had a hard time finding stuff to criticize, there was a good chance they didn't really know the language. or were not particularly critical developers.