1) Leg Curls are almost solely a hamstring movement. Depends how you do them, but usually there are no other muscles as greatly involved. Thus, doing only leg curls for leg work may lead to hamstring being more dominant than quads and glutes. If you add curls to your program, it will strengthen your hamstrings, which doesn't immidiately cause some kind of dominance. Most people could use some more hamstring work.
2) Isn't natural? Flexing your knee isn't natural? Weren't you a runner? You should know this. It simulates the flexion of the knee. You can do leg curls when you're standing also. Maybe then you'll get a more realistic view of things.
Knee flexion is a passive motion during sprinting and walking. Some coaches warn against the use of leg curls because they say they can cause a person to become dominant with the hamstrings during hip extension.