Really, please, tell us more!
It's not that you're wrong, it absolutely is a really good way of finding out how someone reacts to being asked to learn something new in a stressful situation. And that is a valuable thing to know - I love working with people who get excited about an opportunity to learn something.
It's just that you have to be prepared to give so much help and the question has to be so easy because it's all completely new that you can't test competence in any meaningful way. Which means that at the end of the 90 minutes or whatever, you know exactly one thing more - their attitude to learning under pressure, but you have relatively little time with them, and you probably need to learn multiple things per session.
You also have to be careful when in the interview you throw this. A bad hit like this can throw a candidate off for the rest of the interview even if they are quite acceptable.
Even being upfront about our low expectations and providing very friendly and helpful people to pair and guide, it lead to record numbers flipping the table and stomping off premises before the completion of the interview.
I don't use this approach now, as I don't think its a good use of time (and I think an interview should be a positive experience even for those that fail), but it did provide, I believe, a fairly strong signal of the attitude with which a candidate approached learning new things (but not really anything else).