Nash needs lots of opportunities in order to convert. He's good at creating chances for himself, but not good at bearing down on them. Do not try to deny this--the evidence from the last 3 seasons is right there. He's certainly not a "shooter" or above average finisher anymore.
Nothing wrong with having a 20+ goal scoring, big bodied presence on the 2nd line. Do not try to deny the fact that the 2nd line has played very good with him on it - the evidence is right there. While he may not be producing at a higher rate, his presence alone is enough to give his linemates room as Nash still gets a lot of attention from defenders.