Why do you think writers use names like ads ? This is a quest for identity by people that have essentialy been made transparent and ignored by society. They reclaim urban space. "This is mine too, it has my name on it". "You can't do as if I didn't exist".
Being the target of such vandalism is no fun, for sure. I have been. When I did, I tried to remind myself tags are scars forgotten kids draw into average and blind people's landscape.
In this world of writers Banksy is a pacifist. He's addressing average people in their own language. He's talking to us with subtlety and good points, he's touching us, instead of shouting on us.
Be it him or any other writer, don't kill the messenger. If tags are pimples, then our society is sick. Would removing the symptoms cure the disease ? I doubt it, and I guess you do too. We know this all too well for a long time but still don't act on it as a group. These are only reminders, or first symptoms, of what is to come if we continue to indebt ourselves to other human beings. They'll reclaim.
Of course, tagging in public, perhaps in a hard to reach place, is a separate skill requiring its own practice... I'd just prefer they did it after having practiced the tag in their backyard first. Like Banksy.
The solution is of course to have legal walls. But that would be far too progressive for most councils.
Tagging doesn't require permanent paint.
Legal walls don't satisfy the goal of getting somewhere challenging.
The definition of art is more a philosophical question that varies from person to person, so getting consensus on defining what Banksy is is bound to ask the question. Perhaps that's one of his objectives.