Tom Gilb once said that "everything can be measured so accurately that the result is better than having no measurement at all" or words to that effect. I'm sure he phrased it better. In this case, some users aren't measured, which drags down the accuracy of the measurement.

Even if the numbers are off by quite a few per cent, I can easily see how some site operators might benefit from knowing e.g. that visitors close one tutorial much quicker than the others.