This seems a little high but the Giants do have a need at the position. Brandon Jacobs left the team via free agency this year and Ahmad Bradshaw has had continued foot injuries. This is an insurance pick, one the Giants, the defending Super Bowl champs, can afford to take.
SB Nation's Big Blue View explains the pick:
There are questions about Polk, but this is a need pick. The Giants simply have to try and protect themselves with Ahmad Bradshaw's feet and ankles being such a huge question.
For more analysis of the pick, visit Mocking The Draft.