As I wrote in the Mendelssohn post, and should have repeated here, the Overture is "intended not to retell the story of the play but to evoke its characters, plot, and general nature." So it will have to be cut up to tell the play, because there's not enough of the right types of music in the incidental music to cover the story. I expected that, even though it was disconcerting. Nevertheless I was pleased at Ashton's imaginative re-purposing of various parts of the incidental music. You're right: it gives the music a unity that could not have been achieved by sticking in other Mendelssohn works. What else, after all, sounds like A Midsummer Night's Dream?