I feel like may OP doesn't understand how runningbacks work?
I don't think Rashad Jennings is great, he's fine, and he's been part of a platoon his entire career... and his time here with the Giants is no different from that. So they rotate backs depending on situations, and nobody gets 20-25 carries consistently.
He's an average back, who lacks any kind of top-level burst, but can make one cut and hit a hole. He can catch the ball out of the backfield, but that role now goes to Vereen, who is a great receiving back. Add with his injury history, and this isn't really much of a question.