If you got to a dirt bike (motorcycle) shop worth their salt go in and buy a package of "Clear" Plate Sealer. It should cost around 5 bucks a sheet, and a sheet should be more than a sufficient amount for one bike. Be sure that when you cut out patches to protect from cable rub etc. you round the patches, don't have any corners since they'll eventually peel up....
Now the million dollar question... Why should i bother i like my bike all scratched to hell.
Well yah i kind of agree with you.... BUT
Sometimes you buy a bike that costs you a fortune, my Bullit cost me over 3 grand, i want to protect it and keep it looking good since i spent all that dough, and if i ever decide to sell it i can peel the sticker off and instead of a huge gouge from cable rub it'll look nice and easier to sell to someone.
Also it really depends on the frame, sometimes cable routing is such that it really demands plate sealer on a bike, i had a GT i-drive that i never patched up and i had cable rub so bad it nearly ate through the dog bone mechanism on the rear supension above the i-drive mechanism. Looked like crap, and it probably will eventually compromise the strength of the part after years of mud riding especially.
You can also dress clear plate sealer on your forks and the undercarriage of your frame for those of us who are fast and kick up rocks or have rocks kicked up on us from chasing our pals down. Just depends how far you wanna go and especially handy for people who constantly upgrade and sell off old bikes/parts.