in general, soldiers plunder buildings or try to find some precious things to themself once they occupied that area ( from houses, dead bodies etc. ). For mordor, gathering resources rate is very low ( because most of things are very very expensive ). Thus, this idea came out.
What I mean from " increasing the motivation " some real players ( I have played ) and even the AI don't care to destroy your farms generally. Instead of they are going to your main city directly with siege weapons to destroy you instantly.
Another issue of 4.0 demo is " destroying buildings and castle/camp walls are too easy " ;so it should be change too really