Lord Farringdon,
Actually, 2 on FD is not flavor of the day. Depending on your region, 2 on FD at all times was procedure before Germanwings. Other regions are now adopting it, and, I think, for good reason.
You are absolutely correct that high inputs at takeoff or landing would make correction almost impossible, but at cruise? I don't know. JAL 350 is a good example of a successful attempt by the f/o and f/e to overpower the capt and regain enough control to save lives. Also, and I know it's not quite the same, but the almighty battle aboard FedEx 705 is instructive.
My feeling is that an extra crew member on FD is better than no one at all in a rogue situation. In the first instance, you have a chance (albeit minimal) to incapacitate the rogue and regain control; in the second, you have zero.