Written questions or responses have an inherent buffer or filter. Since speech is so easy, random questions with obvious answers are too easy to fire off. I give my students 5 seconds to contemplate their question, and even repeat it back to them. Often, the answer was right in front of them.
Also, yes/no questions are too easy to answer and it will take longer to deduce that a core concept has not been understood. It might take a student to verbally ask the same question three times before some red flags pop up. However, on a written response test, it's easy to see if a student is doing well.
Unless the program subject is verbally oriented, I don't think acceptable verbal responses will be wide spread.
