Well alot of it has to do with packaging, parts interchangeability, a certain goal they are trying to reach etc., and like with 4 cylinders, sometime it's just something you have to do. The 4.3's didn't start off with a balance shaft but they ended up going to one to try and smooth out vibrations. All crankshafts have counterweights but sometimes the design of the engine and the firing sequence can produce unwanted vibrations. Take the 4.3 for example, it's basically a 350 chevy with 2 cylinders chopped off. But due to the firing sequence, it's a shaker! In a V8 it works fine but in the V6 it makes it lopey. Here's a good read:

http://en.wikipedia.org/wiki/Balance_shaft