With all due respect to the non believers that landing lights create drag, in my company many (another UK airbus operator), many people lower the landing lights for drag. Our policy is when gear is up landing lights are off and when gear down landing lights on (although the manual does state if we want them on we can leave them on - no one does).
Lowering the landing lights on the airbus gives you an extra 100/200 feet per minute rate of descent, which when you are flying a continuous descent can help you just that little bit extra which prevents use of speedbrake.
But why not just use the speedbrake you say? On the A319 there is an anomally where when flap is out if the speedbrake is used then the autothrust will not fly the target speed. It flies approx 10 kts above the target speed, which is a pain in the butt when ATC at LHR require accurate flying... So lowering the landing lights DOES have benefits depending on type and conditions
