In grading some papers today, I came across one where the student was baffled at the fact that an SLP would allow a child 'play breaks' between test sets. I believe that play can be very constructive. For this specific example, speech assessments can be long and taxing. Most children (especially pre-school age) NEED a break. They are not accustomed to sitting for long periods of time. A one or two minute play break is plenty and helps them perform better/more accurately and they are happier.
Play therapy is one of the best ways to engage children in "work." They will be learning and growing and progressing while having fun. Drill is the most effective therapy, but if a child gets drilled to the point that they do not want to attend therapy, they will do worse. A good mix of drill and play is best!