For a good example, I suggest you compare the DH-2, which is excluded from the FG-Addon default listing (because of its 2 rating in systems) and the DH-6, which is included. Think the comparison will make my point pretty clear.
Leaving aside the question whether the DHC-2 is correctly rated in systems, the difference says that the DHC-2 has
* no accurate startup procedure
* no generic failure modeling
* no ability to follow the real standard checklists in their entirety
* no ability to simulate emergency procedures
I'm quite familiar with the DHC-6 and can vouch that it has quite a detailed startup checklist and quite a bit of failure modeling implemented and that there's plenty of checklists available for its operation, so I see the rating of 4 for systems on the DHC-6 rather justified.
I'd guess that the DHC-2 lacks the higher rating because it doesn't really do failure modeling of any sort (?), but I'm not familiar enough to really comment on details. It does seem to deserve a lower rating than the DHC-6 though, whether this really should be a 2 or a 3 I can't say without more details.
I would also like to point out that for a comparatively simple aircraft, creating a 'complete' simulation of systems (leading to a 5) is about a hundred times less involved than even reaching a 3 in a craft as complex as the e.g. Space Shuttle (when the 3 asks for 'no unrealistic systems'). So the rating is not based on 'how many systems in total did you simulate' (if that were the case, the Shuttle would require a rating of 50 or so), the question is 'what fraction of all really existing systems and procedures is supported'.
So it would seem that the DHC-2 is perhaps close to getting a 3 rating, but not quite there.
The short version - I completely fail to see the point you're trying to make, and I assure you there's no such thing as a 'default listing' which excludes anything on FGAddon from distribution.
regardless of whether you have five categories or four.
Kindly read what is written - there's four categories that can be rated with up to five points each - not five categories.