They have much to gain by strength training. If you are a rower, I am not sure what to tell you exactly. The act of rowing training already improves the rower's ability to generate force with both legs simultaneously compared to untrained people. Much of the rowers strength depends on coordination, not just muscle mass. Rowing has a mixed tradition when it comes to strength training.
Some great programs do a lot, others do none.
So the jury is still out. More on all this when I can be more definitive.