Interesting comments here.
GTTM: Do you have a copy of In Search of Christian Freedom? Raymond Franz's book addresses the JWs on this in Chapters 6 & 7 "Salvation, by Faith, NOT Works" and "From House to House" respectively.
The underlying sentiment is how human standards and legalism are the only true forces behind JW field service. What I found of great benefit is how Franz shows why JWs do the door to door even though it goes contrary to what the early Christians likely did. At the end of that chapter one will see how manipulative FS is and how it really is a method for control & burdening (as is JW dogma in general).
I recommend you get and read this book as a historical reference. It's gives background into a lot of JW practices.