Don you said:

"positive farm Return On Cost (ROC) is required to survive, no matter what farm income is."


While i agree with that statement, over the long term(especially as it relates to individual farmers). 

Net farm income shows roughly the same thing(Gross farm income minus expenses), but in aggregate across the entire Ag. economy, not just corn and bean farmers and in any farm discussion about policy, net farm income is used for a barometer for financial health of the Ag. economy as a whole.