Maybe it's just the convenience of the whole thing.

I think that's a big part of it. He's not a good plot element, but he's a good WoW element.

Then what does he do? Goes and chills out in his secret lair and waits for us to go get him.

He's a bad character, but a good WoW character. *points at my reply to Achi* He's all evil. He's got no grey, it's just a black/white thing. That's boring. Lich King? Not black/white, thus a lot more interesting. (Though it took us time to find that out, didn't it? Maybe it'll turn out that Deathwing is actually a kitten deep down inside!)