The NBA is obviously a different issue as you have several small-market teams which are competitive and many large-market teams which are not.
The issue isn't dilution, it's distribution. You'd have a lot more baseball fans in places like Tampa, Kansas City and Pittsburgh if these teams were able to compete on some relatively equal footing for the available talent.
The NBA also has a tougher salary cap than baseball does. In the NBA it comes down to management too, just like in baseball. The San Antonio Spurs are an extremely well-run franchise. The Memphis Grizzlies are not.
But why am I discussing it here, seeing as how I couldn't get an NBA season thread off the ground?