A running back should ideally have good ball security without needing a second arm on the ball all that often. Having a free arm increases the runners balance, feel, and allows him to stiff arm for extra yardage.
Michael has been pretty good in his second stint with us, but I think the praise for his ball security is misplaced - any Joe Schmoe can put two hands on a football, it's a handicap not a skill. Several of his runs could have been longer if he had an arm free to stiff arm or to retain his balance at the end of his runs. While it's nice to see that Michael is obsessed with protecting the football, he still needs to learn how to protect the ball without needing both arms.
Once he learns how to do that, he will be an even more effective runner than he is now.