Wow! i guess I don't brew enough. I also only own 3 kegs but I don't think I have ever marked a carboy or keg. hmm... wonder if I am doing it wrong?
I've got four carboys fermenting right now (of course, I had a three month hiatus so I need to catch up). Two are a split batch with different yeast, so the labels are important for that.
I add %ABV so I can update my "now serving" menu w/o having to go back to Beersmith.
I typically add ABV as well. Especially important at parties, where I've found that the 10% tripels get drained while the wits and ambers barely get touched.