Necas isn't happy playing the role he is because he feels like he could be more productive elsewhere. And he's probably right. And any other player of similar mold is going to feel the same way. Because Rod wants his forwards to be defensively-responsible first and worry about scoring/points second.
Guys like Marner, Ehlers, Necas, Skinner, etc. are great complimentary guys to have because they'll score goals and they'll do it in bunches. But that's ALL they do. And in today's game, and especially in Rod's system, you can't be a one-trick pony.