It's true that the Cover-2 devalues cornerbacks - it's the one position on the field it does devalue.  But what I think made the Bears D so successful last year is that they didn't just stick to a Tampa-2; they were able to mix it up with some blitzes, some Cover-1's, some man-to-man coverage.  Vasher and Tillman made huge plays trhoughout the playoffs that gave our team the ability to cover most receivers straight up, and allowed Rivera to mix and match our defense to the situation and opposition.  In fact, the most haunting criticism of the Super Bowl was that the Bears allowed Peyton to dump off to short receivers too comfortably and gave Rhodes/Addai too much running room for 4-6 yd. pickups.  That's [i]exactly[/i] what the Cover-2's weakness are.  But because we were scared of Harrison/Wayne/Clark, and b/c Manning managed to burn the blitz on their 1st TD drive, the Bears settled into a more passive Tampa-2 defense that allowed the Colts 5-7 yards at a time, kept their D on the field, and squeezed the clock.
If we let Tillman - or worse, Vasher - go, then the Bears' ability to play defenses other than the Tampa-2 will be greatly damaged.  Not only will we be unable to blitz when necessary, but opposing teams will know we are hamstrung, allowing to pick away at the Tampa-2 as they please.  As you said in your mock draft analysis - the way to win is with a great, dominant D and solid offense.  The Colts can win with a dominant offense and solid D< so they can let their corners go.  We can't - not unless we get a Peyton Manning clone at QB (yes, another chance to shill for Quinn).  Otherwise, the defense which we will need to make huge stops will come up just a bit short once again.