You only need a Blender node when you need to blend two colors.  If you only want to affect one value, you can multiply.  If you had two maps, you'd need a Blender node.  If you just want to control the strength of one map, then multiply works just fine.  And since displacement is single channel (as is any control map), you don't even need a color multiply, a regular one will do.  Especially considering render GC affects Blender nodes but not regular math nodes.