__geo_interface__ is "a GeoJSON-like protocol for geo-spatial (GIS) vector data." It follows a similar model/idea as NumPy's Array Interface. It was proposed or put forward in 2013, and more than a dozen geospatial Python applications (see Python Geo_interface applications) implement the protocol. The protocol allows for simple and efficient transferring of features and geometries between various geospatial Python packages.
ArcPy currently supports __geo_interface__ in its Geometry classes (it can be argued there is only partial support since the current implementation is broken in several ways). On a system with both ArcPy and Shapely installed, passing a geometry object from the former to the latter can be done efficiently with a single function call:
>>> import arcpy
>>> from shapely.geometry import shape
>>> arc_polygon = arcpy.FromWKT('POLYGON((0 0, 10 0, 10 10, 0 10, 0 0))',
>>> shapely_polygon = shape(arc_polygon) # pass ArcPy geometry to Shapely
Granted, there are numerous ways features and geometry objects can be passed between geospatial Python packages, but __geo_interface__ represents an established, open, well-adopted, and efficient means that should be supported within the ArcGIS Python API. Additionally, the protocol is lightweight and straightforward to implement.