When Skylab was sent to space, it was meant to be ready for use immediately. Spacewalks were posssible but they were not meant for the growth of the station.
There were no spacewalks with Salyut 1, 3, 4 and 5.
Salyut 6 and 7 and Mir implied spacewalks but only once in a while (unless some kind of emergency came up or a previous spacewalk xas not completed, then there were more).
ISS was meant to grow with many many more spacewalks (this had been criticized by Bill Fisher who lost his job because of running out of favor to Gearge Abbey's eyes, so did Anna Fisher BTW..a big pitty).
This shows spacewalks are not necessarily mandatory and can be occasionnal. It depends on the design of the space station or the spaceship.
About the competences of the astronauts: the Apollo J flight (15, 16, 17) were far more scientific and the CDR's were very well trained into geology (Scott recognized immediately anorthosite on the ground); Schmitt , the geologist, was as skilled a PLT as any other test pilot.
The crews train for years in every aspect of their flight; they can take over the other's job to a large extend.
But here again, it's a matter of design: do we let astronauts fly or do we do it largely automatically? Leaving a smaller part to the human interface, a small enough part so that a scientist can be trained to fly the lander. (Claudie Haignere had trained as a flight engineer even though she never was an army pilot). Do we allow for easy communication with Earth, so that the humans can get help from Earth, even considering the minutes it takes for transmissions between the planets?
In the future, flying a spacecraft should be made easier.
And also: sending an astronaut who's main competences can only show up months after the mission began and only for a fraction of the duration of the mission might not be that seducing.
Help from the Earth, user friendly use of systems, automaticity, computers, virtual reality technology ..all can help astronauts in tasks they had not trained for. The important thing is the training before the flight and the knowledge of their spacecraft, lander, rover.

Raoul