Think of it another way. Last year we had Lynch, Turbin, Washington. Washington averaged LESS THAN 2 carries a game.
Then you add in M-Rob. IMO you can Bring in another tailback or you can use that extra roster spot (knowing Harvin is your #3 RB) and bring in an extra WR, H-back, or TE. I think the fact that Harvin is our #3 back it gives us a TON of roster flexibility
For instance last year WE had 12 roster spots with the skill positions.
3- RB's- Lynch, Turbin, Washington
3- TE's- Miller, McCoy, McGrath/other
5- WR's- Rice, Tate, Baldwin, Kearse, Butler/Martin/Edwards/ect.
In 2013 with Harvin you've got more options.
3- RB's- Lynch, Turbin, Harvin
3-TE's- Miller, McCoy, ???- this one could either be a H-back (Jordan Reed in Draft) or a traditional TE)
5- WR's- Rice, Tate, Baldwin, S.Williams, Kearse
This allows you to keep one more developing WR.