The Director of Bands is responsible for the program. As the head director, I choose to conduct the Honor Band. If I decide to allow one of the assistant directors to conduct a piece, that's my choice.

If the assistant director is requesting podium time with the top band, it is your decision whether that person is ready for that podium time or not with that ensemble. If the assistant director is demanding podium time with the top band, then the answer is absolutely not and we need to have a discussion about program responsibilities and job descriptions. It isn't about ego. It's about responsibility, accountability, and roles within the program.