b55 has a very valid point...
...but there is no reason to make the captain PNF as long as everything is working fine.
When things start to go haywire it may be a good idea for the captain to hand over the actual flying to the co-pilot leaving himself free to analyse the situation and communicate with others that can give him advice and help. After all, the actual flying of an aeroplane is not that difficult unless something like strutural damage has messed it up.
I believe this is more or less what captain Haynes did when his DC-10 spontaneously scrapped one of its engines. He had the co-pilot fly the plane and the instructor that came to help handled the thrust on the remaining engines leaving himself free to handle the situation together with the FE.
When things go very wrong it may of course be a good idea for the captain to handle the controls at some point so that he can feel the aircraft for himself for a while (which Haynes also did).
If the captain insists on doing the flying himself he uses part of his capacity for this thus degrading his performance unnecessarliy.