Geoffery Colvin has written a book Angel Customers and Demon Customers. He has some nice point of view on these kinds of customers:
In our experience across a wide range of industries, companies typically find that the best 20 percent of their customers account for 150 percent of total profits! The worst 20 percent typically lose money equal to 75 percent of profits, while the remaining 60 percent of customers account for the rest. Knowing which customers are angels and which are demons presents an enormous opportunity.
Once you know the true profitability of your customers, you can figure out the reasons behind the numbers. For your unprofitable customers, you'll have to face the reality that you're not offering them a compelling value proposition - a way of meeting their needs so well that they'll reward you with handsome profitability. You'll have to devise new, better, value propositions for them, which our experience shows you can probably do. As a result, you'll start to turn those unprofitable customers into profitable ones, which typically creates a substantial swing in the business's overall profitability.
In the end, you may find that a small percentage of customers just cannot be made profitable. By the time you've figured out who they are, you'll understand very well why they probably aren't worth keeping.”
Have you found yours?
Thro' Brand Autopsy