Knowledge is great: I suspect we can agree there. Sadly, though, we can't guarantee ourselves infinite time in which to learn everything eventually, and in the meantime, there are plenty of situations where having irrelevant knowledge instead of more instrumentally useful knowledge can be decidedly suboptimal. Therefore, there's good reason to work out what facts we'll need to deploy and give special priority to learning those facts. There's nothing intrinsically more interesting or valuable about the knowledge that the capital of the United States is Washington, D.C. than there is about the knowledge that the capital of Bali is Denpasar, but unless you live or spend a lot of time in Indonesia, the latter knowledge will be less likely to come up.
It seems the same is true of procedural knowledge (with the quirk that it's easier to deliberately put yourself in situations where you use whatever procedural knowledge you have than it is to arrange to need to know the capital of Bali.) If your procedural knowledge is useful, and also difficult to obtain or unpopular to practice or both, you might even turn it into a career (or save money that you would have spent hiring people who have).
Rationality is sort of the ur-procedure, but after a certain point - the point where you're no longer buying into supernaturalist superstition, begging for a Darwin Award, or falling for cheap scams - its marginal practical value diminishes. Practicing rationality as an art is fun and there's some chance it'll yield a high return, but evolution (genetic and memetic) didn't do that bad of a job on us: we enter adulthood with an arsenal of heuristics that are mostly good enough. A little patching of the worst leaks, some bailing of bilge that got in early on, and you have a serviceable brain-yacht. (Sound of metaphor straining.)
So when you want to spend time on learning or honing a skill, it makes sense to choose skills with a high return on investment, be it in terms of fun, resources, the goodwill of others, insurance against emergency, or other valuable results. Note that if you learned a skill, used it to learn a non-customized fact, and do not anticipate using the skill again, it's not the skill that was useful; the skill was just a sine qua non for the useful fact, and others don't have to duplicate the research process to benefit. A skill that yielded one (or more) customized facts - i.e., facts about yourself, that you can't go on to share straight up with other people - might be a useful skill in this way, however.
For practical daily purposes, what is your most valuable skill (or what most valuable skill are you trying to attain now)? Post it in the comments, along with what makes your skill valuable, tips for picking it up, and what made you first investigate it.
Thank you for explaining your opinion of the value of becoming bilingual.
The 7-year figure comes from the chairperson of the English Department of community college near me (College of Marin). She teaches mainly recent immigrants from the third world and also people from Eastern Europe who come to this country to study English for a year.
There was a group of eight or so students, mostly women in their early twenties, from the Czech Republic here a year to study English at College of Marin. I got to know them because they would gather for coffee and cake every Wednesday at the same Borders where I played chess every Wednesday. They always spoke Czech when they came to socialize with their friends over coffee and cake: I literally never heard a word of English from their table and did not guess that they were here to learn English till they told me.
Also near me is Dominican University, which runs a residential English-as-a-second-language program. The Czechs I met did not attend this program because it is cheaper to find your own lodgings and take classes at College of Marin. Most of the students in the residential program are (or rather used to be) Japanese and Korean, and about ten years ago, I used to socialize with them, eating dinner at Dominican University's dining hall, playing table tennis with them, hanging out in the library, where they would all go after dinner. I did ask one or two of them if they studied anything besides the English language and they said no.
The Japanese and Korean students actually used English more than half of the time when talking to each other, and they got a few subtle points across to me, but that is more because they tended to be very bright and skilled at interpersonal communications generally than from any mastery in English.
Now I never asked any of these English learners how much English they studied before they came to the U.S. to study English, but one would expect them to have studied it quite a bit because that maximizes the benefit derived from what is clearly a costly stay in the U.S.
Anyway, there is some of the evidence I am using to conclude that to learn how to communicate in another language well enough materially to help you in your career (or to materially aid a social goal like marrying a native speaker of that language) would be more work than getting a master's degree in a science -- at least for most of the people reading these words. But then of course it is quite valuable just to be able to read stuff written in, e.g., Spanish or Chinese, and that is probably an easier skill to acquire.
The foreigners resident in the US have the massive advantage of having the opportunity (and need!) to use English out of class; but they have the massive disadvantage of being adults. A middle school student who moves to another country will become fluent. But a move at the end of middle school will probably result in an accent.
Added, years later: Fluency has nothing to do with accent. It's true that adults get accents. But it is also true that adults learn languages faster than children.