Let's look at it and figure out how to avoid it. Most people rent or buy a mailing list, and they base the list only on demographics. As you probably know, demographics give you zip codes, addresses and annual household income. Judy Diamond's lists go into a great deal more detail, giving you lots of very specific industry-related information. However, other lists are based exclusively on demographics.
Demographics are necessary.
Even the worst list serves as a necessary qualifying element. Unfortunately, demographics are nearly always misunderstood and misapplied. The blunder begins when you look to do business with all those various kinds of people in that list. For many various reasons, they would not be your preferred type of clients. Come on, the economy is weird right now, and that means people are being laid off.