I would avaoid doubling the parts with other intruments, Steve heard this as a trombone feature I wouldn't want to spoil it. Yes it is exposed which is great for a developing band, something as a challenge for the less confident player. Exposed yes difficult no with a little practice. There is the danger by doubling that you are indirectly telling the trombones "you don't trust them". Not the best way to develop a band. We should try and move away from covering all the notesd at the expense of timbre, obviously if there is a trombone missing then cover it with another instrument. If there's a full section then make sure they understand that it is their challenge.