Some Half-Life 2 materials are set to use translucent transparency despite having binary alpha channels. This video shows all of the Half-Life 2 textures I could find that fit this description (and admittedly a few incorrect ones) and gives a direct comparison between translucent and alpha tested transparency modes.
Benefits to Alphatest:
1. No (or reduced) overdrawing
2. Sharp and defined edges
3. No render order issues
Benefits to Translucent:
1. Antialiasing support
2. Allows for smooth gradients around edges