Yep, I think that’s about right. Without the washer mod, the amount of intentional friction between the top of the backing plate and the shroud is pretty small. It’s just enough designed-in rubbing friction to prevent the pad from spinning out of control if the pad is spinning in free air. It does leave some debris on the top of the backing plate due to the rubbing, so it might be a good idea to clean that every now and then if you don't add the washer spacer.
If your pad has a couple rotations per second or more while you’re using the machine, the small rubbing friction isn’t going to make much difference in the rotations. But if you’re on a curved panel and the pad is barely rotating or on the verge of stopping, that small extra friction is going to stop the pad sooner.
With the washer mod spacer, since the backing plate no is no longer contacting the shroud, you have a bit more margin before the pad stops rotating. You can think of it like this: you have centripetal force driving the pad rotation, and various resistance forces countering that driving force to slow rotations.
It’s when the sum of all resistance to rotation (e.g. user pressure on the pad, plus resistance from non-flat contact area, plus the resistance from the pad/polish combination, plus the resistance from the backing plate rubbing on the shroud) is greater than the centripetal force driving the rotation, that the rotation stops. Taking that backing plate resistance out of the equation (by installing the washer spacer) just gives you a bit more margin before rotation stops.
Since I don’t see the need to prevent the pad from free spinning when the machine is running in free air, I don’t see any reason not to put the washer/spacer in, to pick up a slight bit of margin on when slow pad rotations eventually stop. But people do get great results without the washer spacer installed, so it’s whatever you’re comfortable with. As Swanicyouth noted, it’s not going to make or break your success with the machine.
On a different but related note…
Someone asked earlier about doing the washer mod on other DA’s. Most of the other machines already have a clear gap between the top of the backing plate and the bottom of the shroud (that gap is where you insert the wrench to loosen/tighten the backing plate).
It would only be useful if other machines also implemented some intentional friction between the top of the backing plate and the shroud in order to prevent out-of-control rotation when the pad is in free air. I don’t think any others do implement that, but I could be wrong.