Why are commercial airplanes painted at all?
I remembered hearing that paint actually reduces the drag of the plane. Searching a bit I found this Boeing document  describing it.

So the painting reduces the fuel costs, instead of increasing them. Another interesting thing is, that planes are regularly repainted to keep the drag low. Of course there is the billboard effect too.

From the report:


  In response to numerous questions raised by 
  Boeing customers regarding the efficacy of 
  surface coatings to reduce drag, Boeing has
  investigated some possible airflow physics 
  explanations. Possible explanations postulated by Boeing aerodynamicists include:
  
  
  Increased regions of laminar flow due to reduced surface roughness.
  Reduction in surface roughness resulting in lower skin friction drag, when flow is turbulent.
  Reduction in dirt and/or insect adhesion resulting in reduced roughness and hence reduced skin friction drag.