I'm always amused when the Left trots out this tired, old canard.  Clearly, if the Presidential election were a national popular vote referendum, campaign strategies would change.  States that were clearly in one column would face increased pressure to turn out even more votes in a race already decided at the state level.  
Imagine a World Series where the total number of runs over 7 games decided the winner.  Game strategy would change immensely.  And, not for the better.