Does having machined brake surface on rims matter?
Rim walls are machined, if not for aesthetical or marketing reasons, to give the braking surface of rims most regular shape possible, removing small irregularities which could cause "hard spots" while braking, specially around the seam (spot where both ends of the rim are connected together during manufacturing).

Also, it seems to me that the residual grooved pattern improves braking power, most probably due to increased contact surface and possibly dirt clearance issues, without noticeable increase in pad wear, but I have no objective data to support this impression except a positive personal experience with a handful of machined-wall rims with v-brakes.

Hope this helps.