Sometimes you shouldn't. It just depends on what you are doing and how often you are doing it?
The main advantage that diamond belts have over silicon carbide belts is their long life and consistent grind. With a silicon carbide belt, they start off very aggressive and then get finer very quickly so the grind you achieve with the belt changes dramatically over the short life of the belt.
With diamond belts, they remain aggressive over the lifespan of the belt and give a consistent cut rate and surface from the first to the last.
If you are working on many pieces and need to know what surface your belt will give you each time, then a diamond belt is a worthwhile investment. If you only occasionally use a belt sander, then silicon carbide belts are probably the better investment.
You can even mix and match using cheaper silicon carbide belts for rough grinding material and then move to the more expensive consistent diamond belts for the finer touch up work.