Diamond plates that scratch the stones are either cheaply made, inaccurate or of too coarse grit. Many that use plates don't know this and puts a wrong face on the stone, hampering the performance greatly. I have 4 different plates for example to cover everything from grinding to polishing so that I can match the face I put with the plate exactly to the stone.
If it's DMT you've tested, they're too inaccurate. If you haven't used DMT (or Atoma) and the plate was priced at below $200 then you run into that same problem of cupping the stone and creating scratches. Speaking of stray "diamonds" scratching the bevel by the way (again only from cheap or bad diamond plates), anything except tomo-nagura and good plates really scratches the soft jigane and makes it very difficult to precision hone tools that has to be flat.
You loose much of the control and accuracy adding in a rougher stone and especially so if it's a Suita you're using. There are particular types of Jnats though that can produce "steel scratchers" that a tomo-nagura or even a full-size identical stone is great at removing but that's very few stones we're talking about.
People have to keep in mind that Iwasaki-san's "Hamono no Mikata" was written long before good diamond plates reached the market. Much has changed since then but I do understand if one wishes to stay traditional and not rely on the technology of the plates.
mainaman, I wish you to understand that it's especially with hard Jnats that diamond plates truly shines and become incredible tools. The finish and speed that I can work with at after rubbing a 1200x Atoma (never use coarse plates on polishing stones) a few seconds is without a doubt superior. The change is literally so big that the stone itself feels different, which is what happens when you use the stone's own particles instead of another, coarser and completely different stone to get a slurry.
Here in the west we really don't have many options regarding diamond plates, in Jp it's not very hard to find a $300-500 plate. Compared to them the accuracy on ours is really terrible as well, or I am yet to have found a company that produces an accurate plate with a good glide.
Oh and by the way, I can share pics of some Jnats as well if it's of interest.
Aiiwadanii, incredibly clean specimen.
Uchigumori Jibiki. Rare, rare sword polishing stone.