That does sound a handy idea. Are they just numbered throughout a whole run of cable, or are the measured for each individual drum? The latter would be easiest to see how much is left, but I can how it would make cutting and winding reels more time-consuming as you'd need to line up the marks.
How about dual-scale markings like tape measures -- Meters one side, feet on the other?
If you look closely at the picture, you see that the dial has two hands. The black hand is the measurement in inches, 0.1" per revolution. The red hand is the measurement in mm, I think 2mm per revolution.
I've two 100m long reels of 6mm2 here under my bed, and it has "PIRELLI CABLES 1993 BASEC 6mm2 6242Y BS6004" embossed every half metre down the cable. No length markers though (like you get on polyethylene water pipe). Gideon.