Hello, I think the usefulness of being multilingual strongly depends on the employment, activity or simply the daily life of the concerned person. For example my native language is Arabic and I speak French, German and English more or less in the same level of fluency. Although I do definitely not regret the time spent in learning those languages and I am still eager to learn and improve further, mastering only one foreign language may have been much more practical and useful for me rather than stumbling between three languages and never really achieving a perfectly satisfying level in any one of them. But apart from usefulness I think being multilingual is a great advantage for understanding the roots of words, the origins of languages and the relations between them, which I do find very interesting !