I presume that the reason they put this program in place and began to keep statistics is that they perceived a significant issue with volunteers never completing assignments. Yes it would be nice if we had more data on that, but without that data it would be really hard to tease out the significance of this marginal difference.

Volunteers who fail to complete real assignments can be very disruptive to an organization and a drain on its other resources who must put in time on training, review and various forms of hand-holding. So, it is probably not a bad idea for Givewell to do this. Naturally volunteers do not want to spend several hours on a make-work assignment but if they are truly interested in benefiting this particular organization then they should be able to see the larger picture that makes it necessary.