If you want to be a run first team in this league, it helps to have two "#1" type rbs. Besides, they're completely different backs.
Also, just look at this past Sunday. This defense, without Alonso, shut the colts out in the first half, and held them to 14 total points.
Meanwhile, Shady contributed to the offense.
That said, I think that in two years Karlos will be a top back in the league.