A point INSIDE a polygon is considered by $geoNear to be distance zero.
Sometimes it is desirable to treat that polygon like a lake and given a point, calculate distance "to the shore." The same polygon saved as a linestring (first poly only of course; no holes) yields the desired distance behavior.
Since it is not practical to store BOTH reps and we cannot run $geoNear on a "literal shape" a nice feature would be to somehow indicate to $geoNear that polygons should be treated as linestrings for the execution of the command.