John
Thanks so much for the thoughtful answers.
Speaking of the sum of it's parts; if a course is a totally-wooded 25 hole gauntlet, would you tend to loosen the gaps knowing you have to hit so many of them during the round ? When the course was 18 holes, it was about right, but now it's a might unfriendly.
Mando, you are really asking some great questions. To me, this one boils down how much you value variety. It might be that the new 7 holes might all be great as individual holes, and they might be great as a group of 7. But if, when added to the existing 18, they just seem like more of the same, you might want to make some changes.
Every hole should be able to stand on its own as much as possible, but every hole has a role to play in the big picture. If you took the 11 biggest, strongest, best-blocking players in the NFL and put them on the field together, you wouldn't have much of an air attack. And if you put the 11 best quarterbacks out there together, you wouldn't have much of anything.
On the other hand, some courses still succeed by having a "theme" and not having as much variety. It's just a call you have to make. Since you're using the word "gauntlet," I'm guessing players might appreciate getting a little breathing room every now and then. A bad day in the woods -- when you're hitting wood every other shot (or worse) -- can almost make you wish you were back in the office. On those days, playing another 7 tight holes might feel more like punishment than fun.
Being a shade-tree course designer with a tract loaded with lots of natural features, the par 3's were easy. Each hole seemed to have an obvious ideal tee area and a ready-made basket location. The par 4's were much tougher for me to visualize. Every once in awhile we used to play safari, and the combined holes were never as good as the par 3's.
You just made three really important points. If you have a lot of good natural features, then designing memorable par threes can indeed be easier than designing exceptional par fours. It's when you don't have the obvious nice tees and pin locations that the scales tip in the other direction, I think.
Second, when you you're working with a property that's wooded, or even just has a lot of trees, it can be really tough to visualize a 250'-300' hole. And obviously it's a lot more difficult to visualize a hole that's twice as long. So when we say that it's "harder" to make that extra-tasty par three, maybe what we're really saying it's that it's harder to get the land to cooperate. Creating the as-tasty par four will be harder in terms of the amount of time and effort required.
Finally, I hope we can all agree that every good par four should break down into two (or more) decent, fair, fun par threes. But putting two existing par threes together doesn't necessarily make a good par four. There are several reasons that's true, but here's the most obvious. Let's say you want to create hole #78, which plays from the #7 tee to the #8 basket. Hole #8 is probably good because the tee is exactly where it is. When it becomes the second half of a combo hole, odds are that very few shots are going to land right on that tee. (In fact, the #8 tee will often be very hard to hit from the #7 tee, just for safety reasons.) Obviously there are times when combo holes like that work great, but you should consider yourself fortunate when that happens.
Thanks for making me think through these issues. Your questions really helped me clarify my thinking, and I hope my answers are helpful to some of you, too.