Not sure I agree. If you're vaguely musical a lot of the basic percussion stuff is learnable quite quickly.. I'm a cornet player, but wild horses wouldn't drag me near a soprano. Apart from the fact that it's clearly an instrument for nutters I've spent my playing career pretty much specialising in back row playing - no solos and the bare minimum of high notes, which is the antithesis of what's needed on soprano. I have, however, played percussion on a contest stage - St Magnus in Yorkshire - and although I wasn't great I didn't make a massive pig's ear (band finished in the top six, anyway). With a brief bit of training doing stuff like bass drum, cymbal rolls, basic tubular bells, tambourine, triangle etc was perfectly manageable. I'd need a lot more than "brief training" to play soprano in the same piece, despite being a cornet player already!Bands have players that can cover bass trombone or sop parts, or learn to play the instruments.
Percussion is a completely different skill and not so easy to learn for a brass player.
Thanks Pauli.Here's the scaba rules on this which I think covers part of one of the points you raise;
UPDATE: scaba do have some changes planned for these rules so anyone attending one of their contests should seek the current position from the association. The form I have quoted may be slightly out of date. However, the example is still valid as a foundation for change to national contest rules.