The logic of the situation is simple. A pilot can fatally crash the plane if there is one person , 2 people or 5 people in the flight deck. Consider the possibility of the pilot monitoring recovering if for example the PF shoves the control wheel forward with full strength at 100 ft agl. Big fat zero!!
There are many other ways. One pilot could get up and grab the axe and bash the other to blazes then do what he likes. The point is , that's an accepted risk. Or simple pull the fire handles over the North Pole! No way to stop it.
Now we introduce a new risk in the form of a cabin crew! They may be many things including a radicalised religious freak or just a troubled young girl jilted by too many pilots out for revenge. Doesn't matter, they are a risk even if they are just a pumped up purser who has a personality clash with an aggressive captain.
I think whoever said there should be 2 pilots in the cockpit if a cc is to be present is spot on.