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HHKB is the only keyboard layout that attempt to position our thumbs more in the middle of the space bar, and help our thumbs to have easier reach to the mods (command and alt keys) on the space bar row.
I prefer to use my thumbs to reach these space bar row mods so that I don't need to move my hands much. If someone is to add 1u keys at the two opposite corners of the HHKB layout, I can accept that those keys at the bottom most corners should be reached by our pinkies - and as such movement involves our whole hand, these corner keys at the bottom row should be assigned less frequently used functions.