The idea that a board's waist width and/or side cut radius is dependent on the riders skill level is absurd. Waist width is a boot size variable, plain and simple.
The rest of the design aspects of a given snowboard, such as SCR, stiffness, camber profile and flex pattern, can definitely dictate whether or not that board is intended for the expert rider, but stating that larger widths and SCR is required for better riders just isn't true. One of the best riders I know was on a mass production K2 soft boot board with a SCR of less then 8 this season and tore up every trail he was on. For me, the board variables, aside from waist wide, dictate the ride characteristics of the board and help me decide which one I am choosing to ride that day. If I'm going to a small local hill on a busy weekend day, I'll take my Coiler 162 Contra 9 SCR or BXFR 169 8/10/9 SCR. If I'm going to a bigger mountain on a weekday then I'll bring my Proteus 180 13 SCR or my Contra BXFR 12 SCR. This is not a skill level thing.