I am not sure what game you are watching but the Runningback is always the dump off guy. It depends if the other team decides to blitz on that particular down. If the other team blitz's the back becomes the blocker. If the line can hold the extra blitz'rs the RB will roll out for the pass. Sometimes the other team doesn't blitz on a pass down and then the runningback becomes the release valve.
There are very few plays that are built for the runningback to catch a pass. It is either going to be a screen pass or he is 3,4,5 guy looked at in a particular passing down.
We all like to see Willis play but he isn't a wide receiver. He isn't a Eric Moulds or a Lee Evans.