Just as you are able to transfer symbology from a layer file to a feature layer in an ArcMap project, it would also be very useful if label description would transfer as well. At this time, this is not possible. You can populate the label descriptions in a layer file, but they do not transfer over when using the ApplySymbologyFromLayer_management command in a script.
I have a project where I apply symbology from layer files to feature layers. In the legend, the label is reserved for valid times, but I would like the description to indicate what the feature layer means. The best way to do that is through the label description.
For example, in a Legend: Layer Name: Boil Order in Effect. Label: Valid until 8 pm this evening Description: A boil order means that water may not be safe for drinking. Please boil your water before consuming.