I'm not really sure we know what Jonas Gray is. The O-line opened up big holes for him and he often wasn't touched until 5 yards downfield. Also, Indy's D-Line is not impressive. Gray bounced off several would be tacklers with relative ease b/c the Colts are not physical. Gray won't be able to do that this weekend against Detroit.
But depth at RB is important, especially as we approach bad December weather. Plus, Blount adds value on kickoff return.