In my last few years as a JW I couldn't shake the feeling that field service was completely futile and an inefficient waste of time. It hit me that I would have felt much better spending that time volunteering and doing something that would help people in a more immediate, concrete way.
For all the emphasis put on the door to door work, the JWs aren't growing any more than any other religion, and not as much as some. Most JWs know this and are only engaging in it because it's required, they don't enjoy it and are not good at it. Most time supposedly spent in door to door work is wasted going to the meeting for field service, driving to and from the territory, driving around aimlessly going to return visits, and stopping for coffee.
Just think what they could have done if those hours had been spent in actual, productive work of some sort. A church I visited did the following in one month:
Organized an Easter egg hunt for low income children, along with some candy eggs the children found cans of tuna and other food for their families.
Got together and made sandwiches, which they handed out along with protein bars to guys waiting to be hired for day labor.
Took the money earmarked for Easter flowers for the church and contributed to the local food bank.
Many church members play musical instruments, they all practice during the week so they can have good music at every service, which included a five piece band, a choir, and a hand bell group. Other church members volunteer to make food for fellowship after services.
Any of those things are better use of time than field service. The Watchtower knows they this, but they don't care, because it keeps people too busy to think about any of this, and because no amount of time spend in field service is ever enough for "mother", it keeps them feeling guilty for not doing more and for not really liking it and not being good at it.