The number one cardinal rule (excuse the pun) in the draft is to not reach for position. This is especially true in the first round, but the principle is true all the way through the draft. When you reach for position you almost always get a bust.
The Cards have graded out all of the draft options based on football ability, thier fit into Cards scheme, and character. Need does impact the grade but not disproportionately. Last year they drafted Doucet based on grade not on need. So what might this mean at 31? A RB may not be picked in the 1st round.
Even in the most generous grading there are only three 1st-round-graded RBs in this draft. On the other hand there are 6-9 1st-round-graded DE/OLB that are projected to play OLB in a 3-4. Unless a RB of a higher grade falls to 31 the odds are that the Cards are going OLB at 31.