(Maybe they used it in Daggerfall but I've never played it)
I mean, when I first played Oblivion after buying it on the main menu all I was thinking was "Woah, Morrowind's theme in trumpet form?"
Now, even on the trailers they are using that same melody.
Its a wonderful Melody, no doubt. But why do you figure they have reused it 3 times? Maybe they think it is a tradition to the series at this point?