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I also find it useful, now that I understand it.

But I also find it useful to know if the xarray coordinate is a netcdf coordinate or netcdf dimension: "A variable with the same name as a dimension is called a coordinate variable". But I guess that is the same as "indicating that xarray created the coordinate itself", as @shoyer mentioned above?

I personally find it useful - maybe not too intuitive though that the behaviour changes depending on whether there are attrs defined for that coordinate variable or not. I agree some documentation on this would be definitely helpful!

At the very least, we should advertise this feature in the documentation. But given how surprising people find it, it might be even better to remove it...

This is an unadvertised feature, though I agree that it may be more trouble than it's worth -- xarray implicitly creates coordinates like range(n) when coordinates corresponding to a dimension are missing from the netCDF file.

The original intent (see https://github.com/pydata/xarray/pull/245) was for more faithful roundtripping of netCDF files, which may not have all coordinate variables defined. Arguably, it would make more sense to only do this when a certain encoding parameter is found (e.g., {'trivial_index': True}), indicating that xarray created the coordinate itself.