Personally, I like to think of darkness as starting again anew, the unblemished, prepared soil that the tree will grow upon and light as the tree. Darkness tends to be associated with things that are unformed, uncertain, destruction (if there are weeds growing in a field, they must be plucked before trees are planted), contemplation/meditation and that which is hidden (like seeds in the ground). Light, on the other hand is order, spiritual illumination, creation, knowledge, action and that which is visible/comprehensible.

Using these definitions (and remember, different people use the same words to mean different things... I just find this definition less confusing and helpful at distinguishing good+evil from simple forces of nature, but more on that later) both light and darkness are necessary for a person, a society or a spiritual community to flourish. Yes, we fear the darkness, because that which is hidden requires us to let go, have faith, perhaps tear down thoughts or structures that we were comfortable with and begin again anew, whereas light is more certain and clear. However, by this definition, both are necessary. Without light, darkness is stagnant. without darkness, light is misguided and pathologic.

I hold in a separate category, the question of "good vs evil". Where one can define good as "promoting light where light is needed and darkness where darkness is needed" and evil as "promoting darkness where light is needed" and vice versa. In that sense, it is not necessarily true that "in order for good to exist, evil must exist as well", since good is simply the natural and the evil is simply "that which isn't in accord with nature". Just like the universe tends towards entropy and isn't NECESSARILY some cosmic struggle between entropy and anti-entropy, so can good exist without an equal proportion of evil.

I'll conclude with a final analogy: If one uses the darkness to rest and the light to act, this is good. If one uses the darkness to act and the light to rest, this is disordered.

Best Wishes,
-TaoSandwich