From reading the old barbers manual in the wiki, it sounds like the ONLY advantage of a smile is that it helps counter the natural tendancy of a razor to frown (since pressure is typically focused at the center of the blade against the hone, and center spends more time making contact when hone width is less than blade length).

You could use the "gillette" argument that faces aren't square so a curve is advantageous for shaving, but I don't buy it.

If you hone properly and don't create a frown, then I don't see the purpose of creating a smile. It just makes your razor harder to hone. Honing a straight-edged razor could not possibly be simpler. Honing one with a smile is a PITA. I've tried both the recommendation in the barber manual and X and neither comes close to creating the kind of sharpness at the heal and toe that my straight-edged razors achieve.