The value of knowledge and what we should learn

In one of my last posts, I suggested to learn math topics as they will be propedeutic and complementary to programming challenges. I think we can gain useful knowledge and overkill knowledge while learning. I will present a story to fix ideas. I am sure you already heard this story, and now we will analyze it from several different points of view.
This famous anecdote goes more or less like this:

The printing machine of a famous newspaper had started to malfunction. Several technicians were called to find where the failure was, but even after a while no one was able to identify what component needed a fix.
So, the newspaper owner called the printer who had worked for many years at the now-broken press, who was at the time retired.
After he had arrived, he looked at the press and at its control panel for some minutes, then picked up a screwdriver to slighty turn a screw. He said "Now the press will do its job."
Incredibly, the press started to work again, and the worker was thanked and asked for a bill for his services.
The day after, the bill arrived and it was for 10000 dollars! The owner cannot believe the worker had had the gut to ask all that money for one minute of work, and sent back the invoice asking for a more detailed report.
A new bill came the next day:
- $ 1.00 for turning a screw
- $ 9999.00 for knowing which screw to turn

This (totally made up I guess) story shows us the value of knowledge. If you substitute screw with line of code and turn with tweak, you will obtain a perfect programming story. This is because there is really nothing about printing in this anecdote, but only a marketing lesson:

• If you spend time to learn valuable [programming] knowledge, you can be paid a large amount of money. Obvious, but the problem is finding the disciplines which are not at risk of obsolescence: a doctor for example spends many years in education which very rarely becomes obsolete, and he is very well-paid troughout his career. The worker has previously spent many years on the press, gaining an enormous insight of how it worked and how to fix every kind of fault. Knowing how to fix a little, replaceable machine or other sacrificeable tools is not so useful knowledge in this context, just like knowing how to repair a motherboard with a soldering iron.
• You have to market yourself: the newspaper owner knows that calling the retired printer could be the way to fix his machine. If no one knows about your special capabilities, it is difficult that someone will be willing to pay you much. Fortunately, the World Wide Web is now the best showcase you could desire: under the right conditions, billions of people can find you easily, in a matter of seconds.
• If your knowledge is useful but not so diffused, the supply and demand laws permits you to charge more: the worker was probably the only one in the city that could fix the press. For example, if the market is saturated with Java basic developers, you may have an hard time finding projects which fall in your skills range. The more specific your capabilities are, the more sought-after you are, provided that there is enough demand for you to make a living: becoming an expert in Sothern Australia butterflies is not what I would suggest.

What have I learned this year? New languages, frameworks, practices? Is my previous knowledge becoming obsolete in the meantime? What valuable things do I know?
These are questions that I ask to myself before starting a new craft, such as learning Erlang. I mean, it is an interesting language and it is fun to play with, but how much am I likely to utilize it in web applications, given that I currently focus on php? I would rather deepen my Zend Framework know-how to be able to tackle more large and rewarding php projects. To be complete, I should also sharpen complementary skills as web design even if I am usually the backend guy. Disciplines which reside next to my main one give me a greater understanding of the overall picture: knowing what a designer works with helps me to communicate with him and to refine my job to perfectly match his.

Learning new things is important to keep an active mind and to feed it with new patterns and experiences. It is a self-improvement task, and in no way I am saying you should focus on work-related topics only. Just make a selection to keep out obsolete ones and to ensecure you are not forgetting the important disciplines before writing Erlang code. And if you have fun with Erlang, just practice with it! Maybe you will find a use for it in the future. The trick is not to think you are updating yourself on new technologies which lie in your field if they don't, as the time for learning is stretched between many topics today. The ROI for learning Erlang is not high for me and I want to invest in the best assets.
That said, knowing which screw to turn on a printing machine is incredibly valuable - but only if your job comprehends fixing printing machines: if you want to design Space Shuttle's computers for a living, you had better study software and hardware engineering. And even if printing machines could be more interesting than physics, the latter is a more pertaining topic since your computers will interface with a real situation outside the Space Shuttle.
As I previously said, if you want to design the Space Shuttle's computers. Don't be fooled in learning high paying disciplines that you don't care about at all. Work fills a big chunk of your life, especially for [software] engineers, and making sure you enjoy it, it's the best favor you can do to yourself. Even if you are less marketable and less rich, but not broke and still happy.