To hell he is, he's dead right, a neighbor that does that to there house does directly affect your ability to sell your home at a given price. Just because it's not a concern for you does not mean it doesn't affect buyers who actually care what the surrounding area looks like. I had to sell my house for 10k less that I wanted to because the neighbors 3 bourses down let there house fall to shit and have created a very negative vibe at the top of the neighborhood. When people comment to there realitors that they love the home but are concerned about the surrounding properties, that's when you know your neighbors lack of care is negatively affecting your investment.
You don't drop your life savings on a house in a neighborhood that has houses painted like clowns, it sets a bad vibe. You can change your house all you want, but you can't do a thing about the ugly neighbor houses around you. Location location location. You can change everything else about a house, just not that.