People have set up warehouses in various cities that allow the sides of their buildings to be used for graffiti, but if someone drew a very nice picture on you without your consent (i.e. sleeping) then wouldn't you wash it off? It's someone else's property, and a lot of the time it's made up of little scribbles. Some neighbours of mine have had to repeatedly re-paint their wall because people who have no respect for the costs of clean up decided to vandalise it repeatedly.
I know it can look good, but that doesn't excuse the artist's decision to use someone else's property, who they haven't even asked, as a canvas. I realise there can be political/social messages, but why not paint your house? Or paint on canvases and leave those around the city? Last year a bunch of jerks broke into a train station while floods were going on and vandalised multiple carts, some of which were brand new. Some of the artwork is kind of pleasanat to look at, but it certainly wasn't asked for and permission wasn't given. Now the council has to squeeze the budget a bit further to clean those carts off, which could have gone towards funding street-art projects.
I think if the person's a good artist and their artwork is good, they ought to do it on a poster or something, somewhere where they can keep it, show it off and enjoy it. Whether it's good or not doesn't matter though if it's done illegally.
Take a look at this flickr set for instance [link] This is in my country, after the revolution. I think this is real street art. These paintings depict different scenes, slogans, and pictures of famous victims of the former regime. The Muslim Brotherhood painted over the street art with white, making it sound like it was an act of maintaining their country. But the truth is they wanted to erase any trace of resistance, literally.
This is another flickr set, this time of the Palestine wall. [link] As the set before it, it is a form of resistance. Street art is a form of self-expression. Sure, sometimes it's misused. But The law is not just or fair, and I think it shouldn't be illegal everywhere.
It really depends. Definitely not on someone's private property or fence because they have to re-paint it (so you're hurting someone else).
It's totally acceptable to graffiti on advertisements however. Ads are way worse than graffiti. At least graffiti looks nice and doesn't try to shove their products down my throat. And still the assholes who advertise with their massive billboards dare to complain when someone messes with them. They are ugly in any landscape and shouldn't be legal.
I like the idea of walls made specially for graffiti because it makes sort of a place that's designed or "arted on" by everyone and anyone - it's like a very big collab at its best (or colour salad at its worst).
I remember when I was in Germany I went through this pedestrian tunnel that was entirely covered in graffiti - some were works of art and some were tagging, but I thought it was beautiful somehow. While it was still a public place, it was a nice addition. In my opinion of course