It suspect that the usefulness or otherwise of a bolt-holster depends on how you travel with your rifles. I suppose if you only ever drive directly from home to your ground and back without ever leaving the rifle unattended in the vehicle then a bolt holster would not be the slightest use.
I, on the other hand, often break journeys to bits of shooting, often stopping overnight. If I'm away to the Highlands, I generally stay in a hotel.
Under my circumstances, what better or more convenient security measure could there be than to have the rifle-bolt with me while the rifle is secure in car or hotel bedroom?
And what better way to have a rifle-bolt with you than hanging from your belt, unobtrusive under your jacket, in a bolt-holster?