Larry Williams
I have no idea as to the source, context or even trade represented in Warren’s image. I seriously doubt that stone is representative of the size of rub stone found in a typical early cabinet shop. Still, flattening is flattening. I can certainly flatten a board that's bigger than my planes. Flattening the rub stone is basically the same process.

Two stone sharpening isn't new. If you read The Joiner and Cabinet Maker you'll find a workman of old, when working in a larger shop, would be subject to a fine for leaving the rub stone out of flat. Workmen also flattened their finish stones on the rub stone.

The ruler trick isn't new. I had a recent conversation with a friend, Mike Gray, who served a British apprenticeship in the 1950s. I didn’t write down his comment but he said something like; "It's an old dodge. Some people back then used veneer instead of a ruler, but the good craftsmen didn‘t have any trouble with traditional sharpening." If you find you get a some advantage in the ruler trick, you're just not removing all the edge wear on the flat face when using your coarse stone.

Flat stones offer one advantage, you can use your coarsest stone to do all the necessary corrective work and then your fine stone(s) to just remove the abrasive signatures of the coarse abrasive without having to do a bunch of work changing the surface condition of the flat face of a tool. Flat is easily repeatable and this traditional method is fast and efficient.

Abrasives are essentially a collection of small cutting edges. If your stone is slow wearing, those cutting edges get dull. Dressing exposes fresh abrasives and keeps the stone cutting quickly. The slower wearing stones, like grinding wheels, need to be dressed to maintain an effective cutting action.

Diamonds are an amazing abrasive, they'll cut anything. The trade-off is that they're slow cutting because of their shape and they're so hard they're brittle--kind of like tool steel, too hard isn't necessarily a good thing. Mounted diamonds have an additional problem. The matrix they're mounted in, whether resin or nickel doesn't stand up to the stringy swarf generated when working ferrous metals and mounted diamond stones wear quickly.

While on the topic, in an earlier thread there was a discussion of wire edges. While theoretically it's not necessary to create and remove a wire edge on each grit of stone, the wire edge is an important control mechanism to make sure you're where you should be during the honing process. A wire should be created and removed on each grit. It seems to me that many who learn to hone with honing guides don't do this. I don't know if you'd consider this a bad habit or an artifact of learning with honing guides but I know skipping the use of this control mechanism results in many of the problems I see people having when we go out to do workshops.