The theory is that the pin in the cam gear and keyway in the crank gear are primarily for alignment, and that torquing the bolts on the crank and the cam to the correct specifications will apply force to the gears and keep them fixed on the crank and cam. Of course, that would be for a stock engine, and as soon as you start running a lot of valve spring pressure, the bolt torque may not be enough. I exceed the factory bolt torque spec on the crank just to make sure there is a whole bunch of pressure holding that gear to the crank, but sometimes even that is not enough as I discovered last summer after my SOHC swallowed an air cleaner stud. People double pin the cams for the same reason, and there isn't a thing wrong with doing that, but usually it isn't necessary, at least in my experience...
