EDIT: I got underdrive and overdrive pulleys backwards. I see no reason to put an underdrive pulley on our trucks, they just don't rev high enough to blow things out due to over speeding them.
The crank pulley should not be reduced in diameter, only increased if increased idle rpm is desired on all components.
The reason you would want to increase idle rpm on components like the alternator or waterpump, etc. is to get higher performance from individual components at low speeds or idle.
For example, plow trucks can benefit from an overdrive pulley on the alternator as a large demand is placed on the electrical system if an electric hydraulic pump is used. Or in cars with large stereo systems, lots of incandescent perimeter lighting, inverters, or and DC load really. When a majority of use occurs at idle RPM.
Available current from an alternator is directly related to its rotational speed, the faster it turns the more current it can output, to an extent.
In hot climates it can be desireable to overdrive the water pump pulley in order to increase idle coolant flow and fan speed. Power steering is usually not overdriven unless there is another device on the system, such as a power steering drivin plow.
However care must be taken to not choose too small a pulley and over speed the components when the engine is redlined. Even if you never redline it. Because someone else may, or you might in an emergency. Generally pulleys are not changed in size by more than 10-15% max, at least that i've seen, except on specialty applications.