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Are we talking about preventing a seatpost to slide down the tube when we put the weigh of our ass on the saddle? Isn't the required "clamping force" to be applied to provide enought friction totally peanuts?

Absolutely. The change in seat-post design has more to do with limiting the "clamping force" more than much else--- to not deform really thin alu or carbon seatposts. The other major factor, I think, in the adoption of these "collars" is their low cost as they--- like threadless stems--- allow for a significant rationalization of manufacture and montage.

One can perhaps argue with the round seatpost design-- round tube inserted into close fitting round tube (milled to fit)--- but it works relatively well and provides a good range of custom rider fit.