I am not sure that the issue with getting pvp is just down to people being worried about losing units. I think the other issue is timing, I always seem to get attacked when I am planning, or sorting resources, or just moved troops to another army, often it is just easier to pay off. The issue is that one party wants to pvp, the other might not at that time. Also I always find that it is the armies that I dont want to pvp with that get attacked. My sheep army, or transport, so I just pay off and then often dont bother to cancel protection.
I think a pvp flag for each army would be a good idea, so you can flag up armies that you would accept pvp for, this would not stop other armies being attacked but WOULD allow that army to be attacked even if protection is in place. I would happily have my 2nd army flagged all the time.