To know how productive something is you first need to determine what its function is.
If the function of field service is to recruit new members it is probably not very productive. For all the time spent on formal ministry perhaps as many conversions are the result of witnessing to family, friends, colleagues, school mates and neighbours.
If the function of field service is to provide opportunity for existing members to demonstrate and reinforce their commitment to the cause then it is probably quite productive.
It has been argued (by James Beckford and Andrew Holden) that the real function of door to door preaching among Witnesses is to reinforce commitment among existing members. At the end of a week when they have been out maybe a few times trying to tell others about their beliefs if the question arises in their mind: "do I really believe all this?", the answer presents itself, "of course I must believe it, otherwise why would I be out on the ministry telling people about it".