Yes, Graham Harrell might be nothing more than a camp arm. The Packers have shown they aren't afraid of only keeping two quarterbacks on the roster rather than the traditional three.
But given what the Packers do on offense, namely throw the ball, a lot, out of the shotgun with multiple reciever sets, I think Harrell is a very, very interesting project for McCarthy to work with.
For a great overview of the type of program Harrell was a part of in college under Mike Leach, take a look at this from Chris over at Smart Football. The kid may have questionable physical skills, but I have no doubt he can handle McCarthy's offense.
"I firmly believe that any man’s finest hour, the greatest fulfillment of all that he holds dear, is that moment when he has worked his heart out in a good cause and lies exhausted on the field of battle – victorious."