Here's what happens when you don't have electrical light and have to use candles and torches – you get a heck of a lot of shadows. A Wizard of Earthsea is kind of like a shadow magnet: it's just covered in them. Let's check out the very first chapter for some examples:
We could go on and on – we counted over a hundred mentions of shadow/shadows, which averages out to about ten shadows per chapter – but that last example points out something useful: even a "bright autumn" has shadows. Or rather, "a bright autumn" will especially have shadows because you need to have some light to get all those shadows.
So if someone asks you, "Why does this book have a lot of shadows rather than darkness?" you can answer that shadows need light. The Master Hand actually makes this point when he notes, "To light a candle is to cast a shadow …" (3.57). Aha! Shadows are another symbol of the cause-and-effect issue that comes up a lot in this book. (That is, the Balance.) Shadows are caused by something – light – that seems to be their exact opposite. It's almost like opposites are connected to each other and depend on each other.
There's another thing about shadows: unless you jump or fly, your shadow's going to stay attached to you. Archmage Gensher reminds Ged of that when he tells him that the shadow monster is connected to him (4.78).
Let's sum this up. Your shadow may be dark and scary, but shadows are an effect of light (you can't have one without the other). Also, your shadow is a part of you. That's why Ged's main competition in the book is his own shadow. It's a necessary part of him, and it's not something that he can really kill, so he has to recognize that it's part of him.