Not for aesthetics as they’d make little visual difference to 8mm, but as I said above, the lip/spigot going into the wheel of an 8mm hubcentric would leave so little metal at its joint with the spacer’s disc where the actual hub pokes through the spacer that there would be minimal strength in it so could easily shear.
The hub carries the weight of the vehicle so needs to be pretty robust, the wheel bolts just hold the wheel to the load bearing hub - so you need a strong lip/spigot joint on the spacer, 10mm would give you an extra 2mm of metal on that joint where the bore locating lip/spigot pokes out.