Audiences can understand TWoK perfectly without having seen Space Seed (I saw the movie before I'd seen a single episode of TOS!), but it really does enhance the experience having seen Kirk and Khan interact face to face.
Yet you wonder how people can enjoy Star Trek 2009. Your statement above is exactly how I feel about that movie.
Both are probably solid dumb popcorn movies that appeal to a wider audience much more than they do fans, who get bothered by the huge holes.