A Director once said to me "There's no real plan with B-Cam, they're just there to grab shots when they can and hopefully they give us something useable."… I almost strangled him.
B-Cam can be very important to a shoot, but if you're not going to have them do anything important, you'd might as well just save the money send them home. The unfortunate thing is that too many directors have the same attitude towards B-Cam as the one I mentioned above. Unless it's a really experienced operator, who's normally A-Cam, and is doing the show as a favor to the DP, B-Cam should never really be left to their own devices. B-Cam is usually either an operator that doesn't have enough experience for A-Cam, or is experienced, but doesn't have enough of an eye for A-Cam. In either case, their shots are going to be pretty useless unless the DP is actively framing every shot for them, and they spend some real time on blocking.
Anyways, so what happens when you leave B-Cam to their own devices? You get hours of unusable shit that you have to sift through just in case they accidentally shit a nugget or two of gold. Oh, and if you're really lucky, some of it might actually be slated. That kind of shit didn't fly on film productions, no slate, no telecine. Shoot unusable, poorly exposed shit and you'll find a day-call op parked in your spot the next morning.