I don't understand why....
If we wanted a guy that will make some defensive brain farts on a regular basis, we have plenty already. If we wanted an "offensive defensmen", ie. one that isn't very good at defense, well we have a few of those already.
I don't get the point of bringing in Corvo for a large sum of money when we could have kept (and maybe still will keep) Carson and Picard for way less money, and have true rebuilding type players, who will make the same mistakes Corvo will, but maybe still have a chance to grow and improve, not to mention being much cheaper.