Don't forget the moral superiority purpose for field service. Let's reminisce for a minute.
What did you do when people opposed your message? Did something along the lines of calling them "goats", "walking dead", talked about their eyes being picked out by birds because of their "wickedness", etc. You had just bothered someone at their home, uninvited, to talk about something that is considered private (religion)--and they ripped you to shreds for your rudeness and picked apart your doctrines (which you weren't able to refute in any meaningful way). You feel like a big dummy. So what do you do? Call them names. Talk to your partner about how they're so evil and Jesus is going to kill them. Now how do you feel? You got it--morally superior! What better way to make yourself feel above all others than to go in field service?
Typical of cults to keep members doing activities that make them feel separate and better than non-members.