What is achieved by this law is minimal, while it impacts upon the ability of British motorcycle manufacturers to produce designs in a classic British style; in an age when motorbikes are often selected for how they look, anything which inhibits a manufacturer's freedom to design a motorbike to look as they please ultimately inhibits his ability to produce a desirable product. In addition, I believe that any minor negative effects which repealing this legislation may have, would be matched by minor improvements in the ease of policing; such numberplates were a requirement prior to 1975 for this very reason.
As for the old-style car numberplates, it seems bizarre to ban their production while effectively admitting that they are fit for purpose, by allowing them to be retained on pre-1973 vehicles. If repealing this law created all of a dozen jobs, then it would still represent an improvement on the current situation.