Sometimes to make a point, and to make it well, you have to literally make a mark on your environment. When traditional channels don’t fill the needs, you’ve got to take it into your own hands. There’s no doubt that a guerrilla-style campaign can be immensely effective. As an artist and a designer, you can reach your audience on a much more immersive level, and penetrate their lives in a way that will “stick” like other methods can’t.
The issue then becomes whether it’s actually ethical or not to vandalize property for the sake of social awareness or improvement. Is the trade-off worth it? Is the good of humanity heavier on your conscious than a few brick walls or light posts? Is potential property damage truly worth the message you send? Is it worth the risk of jail?
I think, a lot of times, it may not be ethical, but it may very well be necessary. If the issue is important enough, and you’re passionate enough, it’s worth whatever the cost.