IMO the in-between sizes (20, 28, ...) are not necessary. When I did 10 TGUs with 16 I could do at least one with the 24. Same experience with the 24 and 32. When my press 6-8RM was the 16 I could press the 24 at least once. Again same experience with 24 and 32. So you can constantly progress through S&S and ROP with the 8Kg jumps.
That's just my thoughts for men who train alone. Instructors may need more variety for their clients/groups and for women the 8Kg jumps might be too big
I have to correct my own statement a bit
Last weekend I ordered a 8 & 12 for armbars (weights suggested by Dave Whitley) and because I introduced my current girlfriend to KBs
. While I was at it I bought a 36 out of a whim
. Now I have used the 36 for two S&S sessions and have to admit that it's great.
To explain further...with the 32 my hips are almost too strong. If I go for max power (which S&S is about), the KB will come up almost too high, so you might aswell just snatch it -> I unintentionally reduce the power output.
The 40 on the hand is still too heavy for my grip. If I go for max power, the KB will fly out of my hand -> I unintentionally reduce the power output.
I had the exact same problem when going from 16 to 24 or 24 to 32.
With the 36 I can go max power with neither the KB coming up too high, nor loosing grip on it.
It's also better for TGUs.
I think with 4Kg instead of 8Kg jumps, you don't need progression tactics like introducing one rep at a time for TGUs, 2h-Swings with a bigger KB or reduced 1h-swing reps or things like that. After you're able to do 100 swings in ~7-8min and 10 TGUs in ~10-12min you can go right to 100swings and 10 TGUs with the KB that's 4Kg heavier (obviously with more rest).
Although I still say that you don't need the in-between sizes to progress through S&S or ETK, they make progressing much easier. Whether that's a good thing or not is up for yourself to decide. I'm happy with my decision to work up to Simple in 8Kg jumps, but now I'm also happy to know the benefits of the 4Kg jumps and I will use them in the future.