Or a WC athlete that wants a beefier ski for an open set, or to better match their weight/power? To your earlier point, +3cm is likely not as quick to someone at 175#, perhaps, but does it make sense the same 165cm works equally well for a 5'8/175# athlete as for a 6'3"/225# athlete?
I'm not sure Head would go out of their way to make the 168 in addition to the 165 in their RD line hoping to sell to non-racers as a 'carver'. They have the Supershape series and i.Speed series for that. Besides, WC race skis aren't exactly flying off the shelves.
I'm convinced, without knowing of course, the difference in size of 2-3cm's isn't as significant as the flex/construction difference. In "my day" we had SL's available in 200, 203 205, and 207cm's, which were equally similarly close in size, but the real significant difference was in the thickness - the 207 was quite noticeably thicker, and stiffer than the 205, and so on.
These days all SL skis look so similar it's hard to tell, but I believe there is a difference. A 160 appears to be ever so slightly thinner than the same ski in a 165. That said, i haven't actually measured the thickness, but it's not hard to spot an athlete on a ski that isn't "big" enough. A good example of this is a 2nd year U16 that gained 25# on the same 160cm they bought the year before.
Anyway, thanks for the conversation.