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Professional orchestras can play the standards without a conductor and effectively often do in those cases where the conductor is borderline incompetent. I've sung Beethoven 9 for example. The "conductor" (one of the several hundred assistants of Karajan...) bought a good orchestra from prague. He did the whole piece in ones. Nobody watched him. The orchestra members know this piece well and played without him.

Often you'll find conductors where you can't read anything: "Now where's the 1? Well, last bar he did it with his wrist, now he's doing it with the tip of the stick, o wait,now he's swimming (unclear speed) again. Ok, now he's changing his frame of conducting (the area in which a conductor does his movement). Wait, what? Now he's giving the one behind his back. How am I supposed to ... Screw him. I'll just listen to the music and start a little early (you hear the orchestra with delay, as you often only get the reflection from the audience's walls).

Edit: We use the rehearsals to get to know a new conductor and to decide when to actively not look at him and get our marks from someplace else. Often you'll get so confused that it's better to "just do it, it'll be ok."

Also I have seen musicians in the orchestra conducting their colleagues in difficult parts where the "masterchief" wasn't any good. We do the same on stage in those cases.