Have you ever heard of a 5th string RB actually getting playing time throughout the year?
If you count Ricky Williams didn't miami do it last year?
Well technically, Leonard Henry was the #4 RB, because Miami hadn't traded for Lamar Gordon until after Ricky left.
Travis Minor -> Lamar Gordon -> Sammy Morris -> Leonard Henry
Besides if your team/RBs are that bad, what is the point in bringing in a 2nd round RB who is behind in his assignments. Might as well bite the bullet and hope for a high draft pick.