The only problem is their collapsing directivity. I don't mind it so much in the vertical plane, but would prefer constant directivity in the horizontal plane. An interesting read is the AES paper called "Improvements in Monitor Loudspeaker Systems," written by David Smith, Don Keele and John Eargle. The paper talks about this kind of design using a CD horn, and discusses the crossover region, directivity issues and other important features of a large two-way loudspeaker.