It doesn't seem like too much to ask for a couple weeks of summer band, but I have has always been someone that likes to do things correctly and I have never been against doing something for what's good for the group. As far as athletics "motivating better", the motivation is usually if you want to "start on the team" or "be on varsity" then be there.
BTW-unless you are on an extended contract you aren't paid to be there. My first day to report to work is August 9th for teacher inservice. So most directors are volunteering their summer as well.