Like it or hate it, we would not exist at the level we do without marching band. That concert band they want to play in would not happen if not for marching band. I do understand and somewhat agree with some of the argument that in many cases - especially large schools - the time and other demands have gotten a little out of hand, which makes it hard to take a hard line, but personally I think it has to be made. If you want to enjoy one part of the program, you have to put in the time. I personally despise pep rallies (I may be alone in this), but it is an important function that supports the school and keeps us on the good side of the school community.
The most I have done when the student (and usually the parent) has some fairly valid reasons is to offer what a JV football player or varsity volleyball player does. In class, does all assignments, does not do marching band rehearsals, plays in the stands at football games. In the two cases I have had someone take me up on this, both have joined the marching part by the end of September.