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How could someone possibly benefit from the guy who he lost his job too? I'm glad you asked.
Jamie Harper was drafted for one reason: to be the third down running back. With Johnson out of the picture and Javon Ringer injured, Harper would have been the starting running back this season. While that would be great for him in the short-term, it wouldn't be the best in the long run.
With Johnson back, Harper is expected to just be the smash part of the smash-and-dash tandem he'll make with Johnson. His job is to get bigger, stronger and more powerful as a North and South rusher—nothing else.
If Johnson weren't there, he'd need to improve every aspect of his game. He'd need to be quicker, able to cut and move, able to catch, and able to make defenders miss. With Johnson, he can afford to hunker down and specialize at what he's already built to do.
Harper would've gotten a lot of touchdowns in 2011 with or without Johnson in the picture, but with Johnson, Jamie Harper gets to be half of the most potent rushing attack in the NFL instead of being just another nameless, faceless running back. And if you ask me, that's a lot better.