I guess should try to clarify myself a bit seeing that I created a bit of an uproar.
A small bit of background of myself. I did grow up in The Pas, MB, Canada. After I graduated from my highschool, I went to college at Minot State University in Minot, North Dakota and graduated with a BA in Music lastyear. I did play in a British Style Brass Band in Minot, ND called the Brass Band of Minot. I played baritone for a couple of years and 2nd euphonium for a couple of years. I also attended the NABBA Brass Band Camp at the International Music Camp, located at The International Music Camp a few times.
http://www.internationalmusiccamp.com/
I think as low brass players regardless of the situation you may be in, learning both clefs (and maybe even all) can only help in all areas of playing, whether a person is just playing in brass bands, or wind bands as their main ensemble and also if they are doing solo work. I think my solo rep. is about 50 50 for which clef my rep. is. My own arrangements and compositions are primarly in tenor clef with the odd spot here and there which I notated a few spots in BC. When I started out as a musician, I was a trumpet player who switched to euphonium in my 3rd year of playing. In my first year of euph. playing, I did play in TC but made the switch and learned BC b/c I was needed to play valve trombone in my highschool jazz band. When I did switched clefs, I kept up my TC playing and felt comfortable going back and forth even to this present day.
As for my comment about the horn, I'm going off what a horn instructor and player has told me. The actual french horn is a different instrument of what we commonly call a french horn.
Sadly, brass bands aren't as common in Canada and to a certain extent the US compared to the UK. But what I can gather, the brass band movement is slowly picking up in Canada and the US now.