I don't see how anyone could learn to think in 3-D by looking at a computer screen, honestly. Likewise, drawing grading plans is equally divorced from the real process, which is to figure out how to utilize both the cut AND the fill efficiently, and close at hand. Being a shaper is 100x better for that.
I guess it all depends on how you look at things. I remember our first attempt at a 3D green detail. We gathered around the screen and things didn't look too good. One staffer said "I think the program is lousy." I said, I think the designer is. We printed out the 2D plan, and I was able to show them that basically, they were drawing pimply little mounds just a foot or two high above the greens, the rolls on the greens had to be more than the "recommended 2-3%" and the slopes, at least those facing the golfers had to be more like 5 or 6 to 1, not the 3 to 1 they had lapsed into drawing. Also, bunkers had to be twice as deep, etc.
In that case, the picture was worth a thousand words, as I had always told them everything had to be bigger in outdoor scale. That said, given the work to be 3D, we don't do as many greens in 3D early for a design check, believing (now that I am doing most of the design work) that I can draw a contour plan to get what I want. Side topic, but any gca can tell you handing a "finished" green sketch to an associate usually never comes back as drawn. They are usually eager to put their stamp on things and "correct" the plan for me. Yeah, sometimes they do find mistakes and its always good to have a second set of somewhat experienced eyes on anything you do.
I agree that even then, plans get you started but the final design has to be done in the field. There are little tweaks contour plans just don't allow me to draw. But other things, like distances from property envelope, elevations of green above flood plain, or for vision from the fw, etc. can be forgotten when just winging it in the field. As the old saying goes, it is cheaper to move the pencil (mouse) than it is to move earth with a bulldozer. And, erasers (delete/undo key) are cheaper than moving the dirt again if your design idea is not right the first time.
And drainage plans. When Jeff Blume became prez of ASGCA earlier this year, I recalled a story of him drawing a green detail at Champions Run in Omaha. Now, it was a hilly site and easy to route drain pipes, but somehow he, perhaps assuming it was too easy, managed to run a pipe on an uphill grade on the plans. The foreman asked me about it on a field visit, and I made him call the office asking for an explanation and correction, just to make Jeff sweat a bit. My guess is he never took drain pipe elevations for granted again. Yes, feeling shame is part of the learning process, LOL>
As to whether CAD is beneficial, its just a different way of working. I mean, it is possible to keep track of bunker, green, tee sizes, etc. without a CAD program, some of us just find it easier to draw a plan first to get those things basically right early. Not that I don't completely change 2-4 greens per project out in the field, but there is really no reason to guess at tee sizes, locations, etc.
As noted, there are some projects where enviro permits or other factors do require certain things to be pretty close to plan. It is always nice when you can avoid such inconveniences, but not always possible.