I agree, of course, with all of the above, but often an excessively firm landing is really not required (long,dry rwys).
Personally, I'm considered a bit rude because I intervene (not by declaring "I have control", but by "helping" on the yoke) almost always as soon as I have the feeling that the FO is going to make a bad landing.
This for two reasons: I think it's not necessary for the FO to learn by the hard way when it comes to landings, and second because passengers deserve an acceptable one.
Other colleagues of mine purposely let extremely firm landings to happen because, they say, it's a good lesson for the FO.
But from the passengers' point of view it's very, very, very bad!
It's a different story on the Airbus, where the captain won't be considered "rude" if he "helps" on the joystick.
I'm told on those airplanes the FO never really knows if he made the landing or not!
Comments from fbw Airbus pilots wellcome!