The main reason a roller cam is better is that the lobes can be cut much steeper on both sides. This allows the designer to do all sorts of things that a standard tappet cam can't do. By using these steep lobes you can increase lift and decrease overlap, increase flow velocity, or perform other tuning functions that are not possible with a standard cam. They can develop more horsepower without the excessive overlap that makes an engine "lope" and can have flatter torque curves. You may be able to get the same horsepower from a standard cam as you can from a roller but the engine's other characteristics may suffer.