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So are you suggesting that most custom bikes should be designed for an inline post, and that layback or setback seatposts are an anachronism?

Almost, but not quite. The inline might not actually be the best kind of post, so there is room for argument that an offset clamp gives enough advantage in the design at the top of the post that it's worth sending the whole of the tube below it in the wrong direction to start with. However, if you are of the view that the Thomson style is the ne plus ultra of rail clamps, the post should not have a kink in it just because you got the seat tube angle wrong.