Yes, it is a functional change, because I thought the existing was probably wrong.
I adjusted the default behaviour to match PROJ4, because I think that is **probably** the intent of the description in the CF conventions.
I think if we're not happy with this behaviour (which is certainly a bit quirky) **or** we don't want to change the interface **or** if we wanted to remove all optional keys, then we could move this logic into the CF loading rules (Pyke code).
Alternatively, we could elect to *only* support cases where both the parallel(s) and map centre are explicitly specified.  Other cases could then either fail to make a coordinate system, or actually raise an error.

Background:
In my opinion CF is just not fully clear which parameters can be omitted (but it does refer to the PROJ4 definition).  The discussion lists the possible 'map parameters' as :
  * standard_parallel - There may be 1 or 2 values.
  * longitude_of_central_meridian
  * latitude_of_projection_origin
  * false_easting
  * false_northing

The last 2 of these clearly **are** optional in use (e.g. the example given), which makes the interpretation of the others less obvious.  In principle, if the intended behaviour is to exactly match PROJ4 operation, then not even the parallels are required.

The motivating example data (from url above), specifies "standard_parallel", "longitude_of_central_meridian" and "latitude_of_projection_origin", so in that case the map centre is specified (but not false_easting and false_northing).
As yet, I don't know of any other netcdf example data.