As mentioned they are no longer mandatory in The Netherlands. They were
introduced in the 1960s I believe after a string of accidents (some fatal).
Since then fields have become a lot bigger, obstacles a lot less and air
traffic a lot busier. We opted to replace all fences around the airport by
ditches, and deleted them from our fleet to improve on look-out. Also,
modern gliders seem less prone to injure pilots compared to the older ones
where the wire would go into the gap of the canopy front (compare K8 line
of fuselage/canopy to discus and you'lle see what I mean).