I was never impressed with either one of the Cowboys RB's (Barber being the better of the two) I would say that Jones will compete for Marice Morris's position at relief RB. Alexander still has his starting position. Jones will add the same dimension (slashing style RB) as Morris has/had. Jones is an improvement over Morris though, I'll give him that much.
I was more impressed with Seattle picking up T.J. Duckett from Detroit (former 1st round draft pick 4 years ago, with the Falcons I believe) for those 3 and 1, 4th and 1 situations that we just couldn't convert this past season. Nothing like a 255 lb. back hitting the line on 4th and 1 to let you know someones home for endzone dinner!