The Ravens official site made it pretty obvious the Bethel move is likely for comp pick purposes:
"The Ravens have waived defensive back/special teams ace Justin Bethel, a move that likely gives Baltimore a fourth-round compensatory pick in next year’s draft. Bethel had a team-leading six special teams tackles this season and was one of the Ravens’ best players on special teams after being signed as a free agent in March. However, when the Tennessee Titans cut former Ravens defensive end Brent Urban over the weekend, it took away the fourth-round compensatory pick the Ravens acquired for losing wide receiver John Brown during free agency."
Yep, Harbaugh made it clear that it was because of the comp pick. He called Bethel the best ST player in the league.
He probably gets signed by the Pats or Browns shortly