The normal reason for not lubing a speedo cable is that they lubrucation won't fix a bad one. If the cable is lubricated with anything but the graphite-like stuff, it will eventually bind and break from crud collecting in the housing.
Most of the time, if the speedo needle is jumping, the cable is shot anyway and "winding up" in the housing. Lubrication won't fix a frayed cable, or a kink etc. Lack of lubrication is only a factor on very old cars, and usually results in a ruined cable before discovered.
Moot point on nearly all newer cars, anyway, as the last MB with a mechanical cable was the early 124 chassis, I think. Certainly nearly everything else is electronic now, my 85 Volvo is, at any rate, and all GM and Ford products, too!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!