I have had a bonnet strap on all my cars and have had no damage at all.
Perhaps the secret ( I hope ! ) is to clean it regularly, dress the leather and wipe the sheepskin down with a slightly damp, clean cloth.
The buckle area is the most likely to be troublesome but again, just keep it clean.
I have had a couple of occasions to be glad of the strap when I didn't secure the bonnet properly so it can be more than just cosmetic.
