Tom D,
That is a thoughtful analysis. I will do the same tonight (in between reading ethics dox supplied by Tony via link)
Nugent always said that he wanted that same 2-4 "original holes" per course, and that most of the others were adaptations and improvements of other concepts tried and proven before.
Your results are practically the same, although I doubt you started from the 15-3 ratio as being good, and you are comparing a world of holes, not just ones you have designed previously. That said, it just turns out, perhaps, to be practical, or average for all of us in the biz to go back in some way to comfortable beginnings when designing a hole.
On an "average" site I see no problem with borrowing ideas, providing they are "play based" ideas. I mean, a good shot type like a Redan, or the reverse slope green at the 12th at White Bear Yacht Club, or platform green, is a good shot type on most courses, if wind allows. And borrowing "en masse" allows you to create a certain balance of holes and shot types you have found favorable for golf balance.
Borrowing aesthetic design elements like clusters of bunkers, or whatever may not be so successful, without thinking through the look and feel of the site.
Generally, I think your approach is right - if you look at each hole with the idea of doing as little as possible to make it "right" it should be a fairly charming hole, whether there is one like it on similar ground anywhere else. I think the key to design is to design without thinking about those extraneous factors. Just do what is best 18 times in a row!
Now, we can debate the phrase "what is best" as much as "what is unique" but there could easily be a dozen "best ideas" for any piece of golf ground.
To me, borrowing