On plumbing, I know they use flux somewhat liberally, I don't know about a horn - someone should be able to comment on this, I am sure it would create a nice even solder flow, and it can be cleaned from the outside, but I wonder about cleaning flux from "inside" the horn, you'd probably have to have it chemically cleaned (whatever that process is) to get that out.
In horn repair and building you use flux as well. It is an acid that preps the surface for soldering. It enables the solder to flow into the joints. Without it, solder just kinda sits on the brass.
Cleaning is as easy as spraying with a water bottle for me. I use liquid flux when doing soft soldering, and for silver soldering I use paste flux with borax.