Hm... tough one. On the one hand I completely sympathize, but on the other hand, orchestras are about playing in a group, and each person is instrumental (ha!) in helping each other person. Rehearsing isn't just about each individual learning the piece, it's about developing a musical relationship with the conductor and the others in the group, and about everyone working together. If the song is really easy for you and not really easy for everyone else, they probably don't just need you for performances.