At uncontrolled fields, the selection of the active runway should be based on the wind direction.  Although there is no prohibition from making a downwind departure, doing so is unwise and may cause a conflict with arriving traffic, or airplanes already in the pattern.  Recently, it has been observed that pilots have disregarded that a runway is in use and decided to use a downwind departure and conflicted with pattern traffic.
 
Before going to the opposite runway, the pilot must consider airport traffic, the wind velocity, the aircraft's capabilities, and ask why he is displaying this potentially dangerous behavior.  The active runway decision should be the result from consensus of pilots using the airport. To not use the active runway can, at worst, lead to a dangerous takeoff, and mid air collision.  The confusion in the pattern can also lead to other unacceptable risks.