I remember a college orchestra rehearsal that didn't go as well as usual. For some reason, that afternoon, there were more wrong notes and imprecise entrances than usual. At the end of the rehearsal, the director admonished the orchestra and explained why every player should be prepared for every rehearsal.
"You're playing great music," he explained. "Don't do it for your colleagues, yourselves, or me. Do it out of respect for what's on the page."
As justification for playing well goes, that's a difficult argument to refute.
