There are multiple good reasons to let him go, really.
His salary isn't huge, but it's way too much to just fork out to someone who's going to be your 4th corner corner. $2,5 million, if I'm correct. We should be off the hook if someone claims him off waivers, right?
And yes, 4th, since he's obviously not going to beat out Sam Shields, the way he's been playing. At the start of the season, I would've said that he'd come back as a starter. Maybe as a nickel CB if Tramon played well. Shields came out of nowhere, though.
Lastly, Harris was never one of the fastest guys and he's getting up there in age. Having your knee busted twice in two years doesn't really help in that department, either. We have no clue how much he has left at this point. Maybe it's close to nothing.