Youth. 5 years is a world of difference in age when you are talking NFL RBs. And why can't both be around? If AK is suspended, you'd be well served to have 2 other backs who you know can play.
Personally i'd rather see a different type of back seeing as we already have a back that can do what he can on the roster. Let's say MI was gone, then i'd be ok with signing him.