So back to your original question about the tank strap being worth it....
I have one that came with the bike I bought used. I've had it on the bike a couple of times and I don't care for it much. As others have said, the pocket is pretty small. Good for holding a garage door remote, ear plugs, credit card, small stuff like that. What I don't like about it is the way it's mounted. You have to take off the plastic dash thing because the strap sits under it..... then the other end of it ties under the seat. As you know the seat has to be unscrewed to lift off. I'm kind of a neat freak about my bike so I wash it all the time. Every time I wash it I have to take all of the above mentioned parts off to get the leather strap/pocket thing off so I can wash and wax it. Took me 2 times to figure out I didn't want to go through the trouble. My garage door opener goes in the front pocket of my jacket now. If you weren't in Australia I'd sell it to you. I'm sure price of shipping would be outrageous.
The other items people have mentioned would be better at stopping the scratches if that's what you're after.
