The first thing you have to understand about a dimmer is that it is a switch that goes on and off really fast, so you are saving power when it is off (during part of the cycle).
The only argument is how much you save vs the light you get. There is also the factor of bulb life.
Most people use dimmers because they want to be able to dim the light. Saving energy is a secondary but real advantage when they are dimmed.
I also use a dimmer on luminaires that have expensive bulbs which don't normally last too long.
