McClellin has often been mocked to the Patriots because Bill Belichick loves versatile players like this. McClellin can line up in a number of positions and help the Pats where they need it the most -- rushing the passer.
SB Nation's Pats Pulpit explains the pick:
Having picked up two pieces for the front seven at picks 27 and 31 (Whitney Mercilus and Kendall Reyes), my original plan was to address another area of need with the 48th pick: safety. However, my planned selection, S Harrison Smith, came off the board the pick before me, and I decided to go with the best available player. That player, in my opinion, was outside linebacker / defensive end Shea McClellin. McClellin provides a lot that the first two selections do not. Unlike Mercilus, McClellin is more of a natural fit at linebacker. In addition to playing outside linebacker in the 3-4 and defensive end in sub-packages, McClellin could probably even play strong-side linebacker similar to Rob Ninkovich in the Patriots' 4-3 sets. On third downs, Patriots coach Bill Belichick could use McClellin in a variety of different spots. In an ideal world, I probably would have traded up from 48 to ensure getting Harrison Smith. However, considering Smith came off the board right before my pick and the safety talent left on the board, I felt the best decision was to go best player available here, and that was McClellin. The plan going forward is to select a safety at the 62nd spot, and there should be a few options available at that spot, a few of whom include Markelle Martin, Brandon Taylor, and George Iloka.
For more analysis of the pick, visit Mocking The Draft.