Water Catchers
I've played euphoniums with and without grime gutters for a lot of decades and while I can't really tell the difference between a flea fart and a fly fart anymore due to hearing loss, I never noticed a discernible difference in sound qualities.
I'd much rather have clothing that wasn't oil-stained and wet than worry about whatever phenomenon your friend seems to have found with a grime gutter attached to the horn.
U.S. Army, Retired (built mid-1950s)
Adams E2 Euph (built 2017)
Boosey & Co. Imperial Euph (built 1941)
Edwards B454 Bass Trombone (built 2012)
Boosey & Hawkes Imperial Eb tuba (built 1958)
Kanstul 33-T lBBb tuba (built 2010)