Zack Bowman wasn't a great cornerback for the Bears last season and certainly not an ideal starter, but he did an admirable job filling in for Charles Tillman and is a standout on special teams.
His ability to contribute on defense and play special teams is certainly a valuable asset, but the Bears chose to bring back two players without such flexibility instead.
Neither Kelvin Hayden nor Sherrick McManis will make much this season. They're both scheduled for a cap hit of just over $500,000, according to Over The Cap, but that money could've been used to bring back Bowman.
As of this writing, Bowman still hadn't signed with another team, although he was rumored to be close to joining the New York Giants. It's hard to envision him getting much more than $1 million per season, which is why I don't understand why the Bears chose to keep Hayden and McManus instead.
Hayden missed all of last season with a hamstring injury. He'll be 31 years old when training camp begins, has a long injury history and wasn't that good to begin with. He was the nickel corner for the Bears in 2012 and looked like the worst member of the secondary. He also has not—and likely will not—play special teams.
McManis is an excellent special teams player, but it's debatable that he's better than Bowman. Even with the Bears injury issues on defense a year ago, he didn't play much and hasn't seen a lot of action outside special teams in his career.
With Tillman, Tim Jennings and Isaiah Frey as their top three corners, the Bears are still likely going to draft a player relatively early. If Hayden doesn't beat out Frey or a rookie for the job at slot corner, he likely won't make the team.
McManis will likely stick because of his ability in the kicking game. Had they kept Bowman, the Bears would get similar production there as well as someone who can play in the base defense, should a starter suffer an injury.
Bowman would have cost more than Hayden and McManus, but his versatility would have been worth it. Had the Bears retained him instead, they also wouldn't have as much of a need to spend a high draft pick on a cornerback.