Some symbols are useful, because they convey a clear message. Stop signs present a clear message, and I know what to do when I see one. I don't need to ask the fellow who put up the sign what he means by that.
Some symbols provide a more vague message. Che Guevara was a Communist guerrilla who hated black people and wanted to outlaw rock music. His face appears on t-shirts which are worn by people who don't seem to know this. I would imagine Che would be mortified that his face is used to make a quick buck, as if I would care what he things. The t-shirts are marketed as a symbol of rebellion. What the wearers of the shirts are rebelling against isn't always clear.
The same goes for Guy Fawkes masks. Guy Fawkes Day is a holiday in the UK that commemorates the failure of the Gunpowder Plot. Guy Fawkes tried to blow up Parliment and establish a Catholic theocracy. Even his fellow Catholics opposed his plan, and he was caught and hanged. Today, Guy Fawkes masks licensed by Warner Brothers are popular among radicals with both feet firmly planted in the air. I'm going to guess that Guy Fawkes and Anonymous have opposing views on separation of church and state. But again, Guy Fawkes is a symbol of rebellion.
When someone displays a Confederate flag, they might be trying to say that they are from the South, that they support states' rights (even though the CSA Constitution required states to allow slavery), or that their ancestors fought for the CSA, (hopefully their ancestors have accomplished other things in the past 150 years). Or they might be displaying that flag just to upset other people.
It is a waste of resources for state and local governments to display a defunct symbol. And it is a waste of energy for me to get upset when that's the reaction that people seek when they display such a symbol.