An average of $5 million per season might seem steep for a player who has never been a "starter" for the Patriots, but it looks like a fair deal for an important member of the secondary. Harmon played a career high 48.5% of the team's defensive snaps in 2016, typically in three-safety sub packages. Since his selection as a 2013 third round pick, Harmon has emerged as a steady contributor as the team's third safety, consistently earning high coverage grades from ProfootballFocus. He's also displayed a knack for timely interceptions, most memorably sealing a dramatic divisional round win over Baltimore during the 2014 championship run.
Bringing back Harmon will allow the Patriots to maintain many of the coverage schemes they've utilized over the past few years. His ability to play a deep centerfield safety role gave the team tons of flexibility regarding coverages. This was on display during the AFC championship game, as the team was able to use either Harmon or Devin McCourty over the top of Antonio Brown. Whichever of those two safeties didn't shadow Brown stayed over the top, preventing any deep shots to any of Pittsburgh's complimentary receivers, while Patrick Chung operated in the box as a man coverage option against tight ends and running backs.
This contract shows that the Patriots value Harmon as a starting caliber player. While he'll likely remain a "bench" player as long as McCourty and Chung are here, he'll remain a key contributor of the Patriots secondary for years to come.