Surely the main function of conditioner is to make the scales of the hair lie flat, making it feel smooth, look shiny, and more importantly to us, make it less absorbent?

I think it's the same argument as whether you should use fabric conditioner when you wash towels. Do you want them to be soft, or do you want them to do their job and dry you as quickly as possible?

My vote is no fabric conditioner on towels, and no hair conditioner on shaving brushes.