I think it’s interesting that the student came to you specifically to ask this question since you are well known for your sketching talent. Don’t you think he got the answer he was expecting?
Anyway, I’ve hired several designers who can’t sketch at all and they add tremendous value to what we do as an experience-design practice. We use many modes of narrative to communicate intent to various audiences in the fuzzy-front-end. I frequently have to throttle traditional Industrial Designers who tend to “over design” early on, thereby ending conversations instead of starting them. But I do think sketching plays its part in post-definition phases.
I think if you asked Gerhy, he’d tell you that rapid model-making trumps sketching, but they compliment each other and sketching tends to “spark” the model making process.