I don't think that's a matter of one answer fits all as that's seldom the case anyway, so I'll try to approach the situation from many different angles.
If one thinks it's legitimate to - without authorisation - paint someone else's walls because "private property is theft and an illusion", as much as I'm an anarchist (or a least that's the closest someone could define me), I can't simply concur with that out of respect for people who don't share those same beliefs. They work hard to earn some money and paint their houses just to wake up the following morning with unwanted scribbles on his newly reformed wall.
In this regard, people who deem their grafitti as unconventional art that should be respected and praised as an enlightening means of communication, actually strike me as regular sociopaths rationalizing why their crimes are completely justifiable. This goes independently of how well executed the job must have been. Most people don't want to step out of their houses and realise someone had painted a perfect yet unauthorized Mona Lisa replica on their door.
If you are one of those who would enjoy a random picture or message painted on your house overnight, don't speak for everyone: just hang a sign outside saying "please grafitti my whole house". Plus, in my country (and probably others) it's illegal to have the front side of your house in a bad shape, most especially the sidewalk and the walls. The owner could be given a fine for that. Therefore, not only it's beyond selfish to damage property and let others pay for that "art", it's inconsistent with any sociopolitical messages that said artist would be trying to portray in his work.
When graffitti is performed on public spaces, the "artist" could argue that said space belongs to him as co-owner and he's entitled to do whatever wants. Problem is, the space also belongs to every people out there and they don't want see images and sentences they don't like everywhere around them. Above all, symbols that could be deemed as demotivational and outraging to some sensitive people are an act of aggression because those people don't have a choice but to forcefully see disrespectful messages displayed in every public place. Said artist is again, being totally inconsistent with his message by merely causing babyish trouble when he should be promoting mature insight.
Some would argue that true art is provocative and subversive otherwise it can't be art. I say if one is trying so hard to be perceived as a grand artist that he doesn't care that his work is actually causing problems rather than self-reflection, he's no artist. Again, that's more like a criminal thinks than an artist. Firstly, artists should do things for the sake of art itself, not to purposely please or otherwise shock particular audiences. Second, if there's a statement in the art but its creator refuses to go through to the ultimate consequences to support his views - like ending up in courts of law - he's inconsistent with his artistic statement. He's a coward and an hypocrite. That's not what an artist should be like, at least in my opinion.
All that said, now let's just assume for a while that the graffittis in question were performed in a totally legitimate way. Or to push it further, let's disregard all the ethical and lawful implications. In that case, the graffittis are left for the viewers to see what they make of them in a totally subjective fashion. The artist has no say in that, has no control. Once it's out there it belongs to no one else. Exactly like music.
Conclusion: I am totally in favour of street art, graffitti or whatever goes along these lines. People just shouldn't do that as a means to harrass everyone just to call attention upon themselves, using the good name of Art to justify their most childish impetus, damaging statues and houses in the process. That's not what art is for me. Rant over.