Again, I don't see what is learned from this that can't be learned some other way. Furthermore, it seems like a waste of time. The time could be used to ask some other question, to allow the candidate to ask questions, to walk around the office (a break from the interview can release tension), to return to a past question the candidate might have had trouble with, and many other things.
BTW, Seth Godin has posted a very controversial entry in his blog about how interviews are a waste of time. Some of his suggestions, such as hiring someone for a weekend, aren't practical for all jobs (e.g. if the candidate is from out of town, or working for a competitor). But he does make some good points that in general, you don't know how well things will work out with your candidate until the candidate becomes an employee.
Note for usenet fans: Chuq von Rospach, one of the major contributors to usenet source code and newsgroup organization back in the day, responded in his own blog.