This can be a religious discussion - something like matching the color of one's purge button with one's wetsuit trim...
I use them on every tank I own (except the pony - can't find one the right diameter) for differing reasons:
For my AL 80's, they protect the tank from dings.
For my Steels (Faber & PST), they protect (somewhat) the other contents of the trunk of my car from the tanks should they break loose and roll, and also protect the (expensive) Nitrox bands from the cam.
Maybe my cam straps are a bit strange, but I stopped having tanks slip when I bought the protectors - they adhere better to the tank surface under pressure than do the straps, and also they adhere nicely to the straps.
As far as asthetics I'd just as soon take them off and show off my beautiful tanks, but frankly they're functional and protect my (expensive) equipment nicely and make it easy to identify yours truly among the swarm of folks wearing the same color wet/drysuits, so they stay on.
Tom