When is a MOSFET more appropriate as a switch than a BJT?
BJT's waste some current whenever they're switched on, regardless of whether the load is drawing anything.  In a battery-powered device, using a BJT to power something whose load is highly variable but is often low will end up wasting a lot of energy.  If a BJT is used to power something with a predictable current draw, though (like a LED), this problem isn't as bad; one can simply set the base-emitter current to be a small fraction of the LED current.