Here are a few more things to ponder.

Horns allow a greater mass of air to be coupled to a driver. For instance in the case of a 6" driver the mass of the air that is actually coupled to the driver is typically much less than the mass of the driver. This causes the driver to behave badly. A well done horn provides an 'impedance match' that allows the designer to couple more mass to the driver. There is a school of thought that states this is what a good horn should do.

There are other ways to control this 'behaving badly', but they all have trade-offs.

For instance a bass horn, well done, will be absolutely enormous due to the huge wavelengths involved. Most bass horns are somewhat compromised (undersized, folded, etc.) due to this sheer scale involved, although they often produce decent results.

And to really throw a curve, there are a lot of folks who like a design called a 'back loaded horn' where a horn is used instead of an enclosure to load the rear of a direct radiator (or front horn) system.

http://hornet.hi-fi.hr/HedStart.htm
http://www.vt52.com/diy/diypage/hedlund/hedlund.html
http://melhuish.org/audio/horn.htm