Yes I understand that football game strategy is complex but the simple fact is that a good defense will improve the running game more than a good running game will improve the defense.
Look at this
Running backs are most useful when their football team has a lead and they can run and use the clock. A team that runs a lot in the first half to establish the run will not be successful in the NFL. Here is a good article that busts the establish-the-run myth.
http://sportsillustrated.cnn.com/2008/writers/michael_lombardi/05/14/nfl.myths/index.html
Also, running success is one of the lowest factors when it comes to winning. Passing success is actually more important.
So we should focus more on our Pass Defense. I will concede that more rushing attempts means fewer pass attempts and that will increase the chances of winning slightly. But that does not warrant choosing a RB in the first round. Just run 5 more times a game.