The Patriots big needs heading into the offseason were receiver and pass rusher. They signed Brandon Lloyd to a multi-year deal so pass rusher remains as a need. Mercilus can be viewed as a boom or bust prospect considering he doesn't have multiple years of good production in college.
SB Nation's Patriots blog, Pats Pulpit, explains the pick:
Heading into the 2012 NFL Draft, the Patriots desperately need to add a dynamic pass rusher to their front seven. Mark Anderson, the team's co-leader in sacks in 2011, departed to the division rival Buffalo Bills on a contract that the Patriots determined was too steep to match. His pass rush counterpart from 2011, Andre Carter, remains an unrestricted free agent on the open market, as he is still recovering from a quad injury that landed him on injured reserve late last season.
The Patriots will likely want to add pieces at wide receiver, running back, interior offensive line, and 5-technique at some point in this draft. However, at 27, Illinois' Whitney Mercilus possesses too much value to pass up at a position of need. While some are labeling Mercilus a "one year wonder," there's no denying his talent or explosion as a pass rusher. He is relentless getting after the quarterback, and is always playing all-out in effort (unlike another prospect I considered, Penn State DT/DE Devon Still). He still has some work to go on his technique, but the athleticism and potential are definitely there. He has the size/speed the Patriots look for in their outside linebackers (6'4", 261 lbs, 4.68 40 yard dash), and even if he did not work out as an outside linebacker, he'd still be able to play a majority of the team's snaps in four man fronts and sub-packages. At some point, the Patriots need to take a risk on a pass rusher who has high potential. Mercilus would be the perfect candidate for that.