In criminology there is a theory about broken windows in lower socio-economic neighborhoods. The theory states that monitoring and maintaining urban environments in a well maintained order can lead to a decrease in overall crime.
For example, it is accepted in most theoretical circles that a house with broken windows is more likely to have more windows broken than a house with no broken windows. This can escalate to other crimes such as vandalism, trespassing and further criminal damage if the broken windows are not repaired.
The same thing goes for tagging graffiti. If a community is vandalized by taggers and they chose to do nothing, i.e. cover the tagging, it can empower the taggers who may escalate their criminal behavior because they know the neighborhood does not care.
The next time you are searching for a home pay attention to homes and businesses with broken windows and vandalism. What does it tell you about the community you may be soon a part of? If the neighborhood does not care about a few broken windows or graffiti, they certainly do not care about property values.
As a side note, if you see graffiti in your neighborhood contact your local municipality. Most cities have some type of graffiti removal department. It is imperative that graffiti be removed within 24 hours in order to send a message to taggers that they are not welcome in your neighborhood.