When we make a judgment, we are saying that one thing has a larger value than another. We have a value function in our brains that takes two arguments and returns whether the first is less than, equal to, or greater than the other. As cruel or as crazy as it may sound, that function can take any two things and compare them; we have to make decisions like Value(CoolJob, CloseToFamily) all the time.
Many people have an instinctive revulsion to the idea that such a value function exists, but it is important to understand that it does, no matter how much you'd like to avoid it. If you are in an improbable situation where you are forced to chose which of your children lives, you will have to make a choice. Refusing to choose is itself a choice, and is a piece of your value function.
In programming, as in life, people often end up using a value function provided by somebody else, rather than actually deciding what they value and what gets them the most with their resources.
Many people are peddling value functions. When someone advocates a methodology, they are also selling you on the value function which their methodology theoretically maximizes. When someone advocates a language or platform, they are selling you the value function where their language or platform has more value than any other.
The correspondence of these value functions to your true value function varies widely. If you use a value function that is greatly at odds with your true value function, you will invariably end up with less true value than you could have gotten. Simple proof: If you choose something you accidentally overvalued, then you pay the opportunity cost on the additional value of what you should have chosen. If you undervalue something, you are likely to end up choosing something else incorrectly and getting less than you should. If you choose poorly enough, you may end up with negative value, even discounting opportunity cost.
It's one thing to choose Java because it's the best choice for you. Maybe you value the cross-platform support. Maybe there's a Java-only library you can buy that gets you closer to the goal in one step than you can get in any other language; that can be valuable. Maybe all of your developers already know Java and that outweighs the costs it has.
It's quite another to choose it because "everybody uses Java", without analysis. Java's got some serious disadvantages, too; are you so willing to accept them without thought?
As you might get from the somewhat slanted tone of the last two paragraphs, I think the "use it because everybody else is using it" heuristic is one of the worst value functions you can adopt. Life's just not that easy.
I can also now re-express the goal of this book more precisely. My goal is not to try to provide you a value function; my goal is to help you build your own. Step one is realizing you need one. Some elements of my personal value function will shine through in this work, but that is because it is unavoidable (and therefore I don't try to hard), not because I truly want you to adopt mine. One of the important ways I can do that is help you consciously think about your value functions.