Why not James?

I understand the FO's excellent personnel choices thus far, from signing McCown to letting Revis go, so I wonder if their infinite wisdom is at work again in regards to why the terrible Martin and mediocre Rainey are our 2 primary ball carriers.  In the limited time that we saw James he performed quite well.....why not give him a shot, it's not like we are gonna wake up Sunday morning and Martin and Rainey will somehow become good all of the sudden.