With respect, I disagree. I've used it on many projects that have never seen a single minute of video to help me work through design stuff or, just help me find that missing measurement that I can't figure out in my head.
Case in point.. The miter station that I just built (videos coming in a couple months), I wanted a little different measurements than I used for mine. Different saw and no systainers being the biggest reasons. I didn't need any "joinery" information but, I did need to figure out the proper width of the modular cabinets to get to the final dimension they were looking for. By doing it in Sketchup, this was a quick and easy change to a single cabinet dimension and then copy/paste to get the entire bench. Also, I neglected to account for the swing of the saw in the initial design and was able to quickly re-design the saw table. I didn't know what the final width was going to be but, I just kept stretching it in Sketchup until I got the clearance I wanted and then pulled a measurement on the drawing which told me what size to cut the shelf.
Additionally, I've used it to figure out is some particular joinery would work (think double domino) in a particular joint or just used it to draw a quick picture to illustrate what I'm trying to explain here.
Now, with all that said, I think if I were starting fresh with design software today, I'm probably go with Fusion 360. I think it's more powerful and probably a little easier to learn.
I do agree... It's a "use it or lose it" skill.. I've found that spending a few minutes each day drawing something - anything - helps! Fortunately, I've had quite a few projects lately that require me to figure something out..
For the record, all my projects start with me in Sketchup before sending to Mark to do his real magic with for the plans.