Well, Helm... That's what tagging is, yes.
Luckily, larger pieces aren't tags, so they usually don't serve the same purpose. Graffiti can be horribly ugly when people do it... wrong. Or out of selfishness. But when a real artist puts his piece on a strategically selected wall, it's artwork, and he paints it not for himself, but for people who are going to see it. I've seen both types, but they're both rare up here. Tags are pathetic cries for attention. But there are some full-wall pieces in a larger city I've been to a few times that continue to blow my mind. And they fit in. Not because it's junk in a junky neighborhood, it's a very tidy and pretty town. And in the middle of it, in an alley that's still very visible from one of the streets, there's a cascade of well-arranged colors in a vaguely letter-like patterns that people look at as they pass.
Me, I've had fun messing with the thought of being a graffiti artist for some time. But what really sparked it was when some kid, probably new in town, started painting these random white lines and curves on a couple of walls back in my old town. One of them was on the side of my apartment. Likewise, on a much more visible street here in my new town, there's a huge concrete wall where some youngsters have been writing offensive things about each other with spray paint.
Painting that stuff over with something that could actually resemble art just seems like a good thing to me. Hard to vandalize something that's already shock full with badly written insults.