So to put it in a very easy to understand reasoning. Compressing the stock spring farther only changes how the spring behaves when the strut is at full extension. When you have weight on the strut it will compress the spring to the same compression as it would have prior to adding preload to the spring. A spacer actually will allow the suspension to travel outside of its designed geometry which is what causes the rougher ride. The bilstein 5100 does not travel outside of the stock geometry because it is still stock length. You trade height for wheel droop with the bilstein or other methods of preloading the spring. Also with the spacers allowing the wheel to travel farther down you run the risk of destroying the cv axles or front differential because they can bind up. On a truck with a solid axle like
@MADDOG has the spacer spacer is merely there to make the spring longer.