A320/319 lights work well as additional speedbrakes, as Tallboy says, they help give you an extra rate of descent if needed, those of you who say this is rubbish might like to try it !
Ezy prefer flap 3 landings on runways which, after carrying out a landing distance calculation on the LPC, are suitable. This depends on factors such as wind, best exit, taxi distance, use of low autobrake etc. Obviously on a short or performance limited runway flap full would be used (unless required by QRH) when a bit of extra brake wear would not be a factor.
The drag lights create does use extra fuel which when operating 1000+ flights a day adds up. There are airmanship arguments for and against light use which is why Ezy does not insist on when they should be on or off but allows the crew to make a decision according to the conditions.