That's a tricky one.
On the modelling side it gives much more choice, hence more potential builders for a gb.
However, GP for example are bikes designed and built specifically for the race track and have never been, nor will ever be, civilian in use, it's like allowing F1 cars in a civilian car build group.
Building a civilian bike as a bike modified for a racing class however would be a different thing as they were civilian bikes to begin with, much like rally cars being modified civilian cars and technically still driven on the road.
If they're to be allowed then the title of th gb really needs to change to motorcycles and lose the civilian tag, personally I think there's enough variety to do a civilian gb and a race gb