I've always thought Cliffs of Dover was an awesome song. I agree with the general message of your post, though, and I do agree that GH3 is almost worth getting just for that song. It's certainly worth a rent or borrow: I've played the song and it rules.

I just don't feel that the remaining points in your first paragraph are enough to outweigh it. There's plenty of good songs in Rock Band to tide me over until Cliffs of Dover is released as a download track, or until GH3 makes its way to the bargain bin.