Going back to the original point......
Why do we have so many different categories?
Well they are continually being invented primarily for 2 reasons, 1) for Airworthiness and 2) for Pilot Licensing.
IMHO we have rather too many ways of doing things.
Firstly, airworthiness:-
There seem to be a number of different categories under which a design can be approved but the two basic divisions are an ICAO compliant C of A or a National system such as the Permit to Fly. If it's ICAO compliant then it's accepted by all ICAO contracting states for flights in their airspace, if it's National then it's up to individual states to accept or otherwise.
It would be helpful, in the absence of ICAO agreement, if National requirements were harmonised as far as possible, however this is the wonderful world of aviation where standards are anathema.
Not unnaturally there are different design approval requirements depending on the physical characteristics of the aircraft, however, whatever method is used to arrive at design approval, the overall goal is to arrive at a design that is airworthy within its operating limitations. Similarly the objective of the continuing airworthiness system of inspections is to ensure that the aircraft continues to be airworthy.
If it's airworthy, then the risk of an inadvertent plummet is acceptably low to the certifying authority, so why can't a Permit aircraft fly at night, over a built-up area, or under IFR? If it's safe enought to let loose in the sky then why the restriction? The aircraft has no idea whether it's dark outside, there's a town underneath, or if the pilot has elected to fly under IFR so it's no more likely to plummet.
Pilot Licensing is another ball game altogether.
Mike