The head of the English department of my local community college (in the US) told me that it takes about 7 years of study for an average immigrant's English to get good enough that native speakers start wanting to have conversations with the immigrant. Some people require much less time, but for most people, learning a new language (particularly learning to speak it and not just read it) is a very large investment in time.
Some claim that learning a second language will make a person better at thinking -- or confer other broad advantages, but I am unconvinced by the evidence I have seen for those claims.
To improve him or herself and his or her society, the person would do better to spend the time he or she would have spent learning a second language learning science, history, technology and practical arts.
Switzerland wants its young people to study both French and German because it wants to remain one country and to avoid splitting into a German-speaking country and a French-speaking one. That is an example where it makes sense to advocate and to encourage bilingualism IMO. I have a lot of sympathy for the Swiss policy of encouraging bilingualism. I have much less sympathy for the general belief that bilingualism and trilingualism are good in themselves. Human lives are short; people should focus their learning on the things that most advance themselves and their societies.
I guess you could argue that all of Europe is in a milder version of the Switzerland situation: that it is necessary for European individuals to invest a lot into learning second and third language to keep Europe integrated and to prevent wars from breaking out. Maybe that is the root of the belief among Europeans that it is good to learn multiple languages.
But I can't help notice that if there were a button that when pushed, makes it so that all of Europe (or all of the world) magically becomes fluent in the same language, I would vote for pushing the button: the advantages are great (namely, freeing up time currently devoted to the learning of second and third European languages for learning more potent things) and the risks are low IMO (the main risk being the small probability that different languages really do confer on their learners significantly different cognitive strengths and weaknesses).