The Boston Symphony has a long tradition of offering the public access to occasional “open rehearsals,” and the events have historically borne less resemblance to an actual rehearsal than to a casual performance. In fact, on the occasion that a conductor or soloist has actually attempted to use these scheduled services to work on a piece of music at some level of detail, the BSO has been guaranteed to receive multiple letters of complaint from those patrons in attendance. Still, new music director James Levine is making it clear that a rehearsal is a rehearsal, and he has no interest in plowing through repertoire for its own sake.