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"What to do about this sickness caused by other people’s wealth? Swingeing taxes on the rich, or smaller differences in pay in the first place, say the authors, citing Sweden and Japan as instances of the two alternatives. A decade ago even left-wing politicians were “intensely relaxed about people getting filthy rich”. Now, as it becomes clearer that some of the rich got that way by theft, the idea that they have also caused injury more subtly will gain a readier hearing. "

I came across this passage in The Economist February 28th issue.

Writer has used 'filthy rich' in this article.
According to Longman and Oxford dictionary filthy rich is an informal word. If that's so, then why writer has used it in a reputed magazine? A magazine like Economist comes under formal category, right?

When a dictionary says that a word is informal, where can we use that word? In which settings and conditions? Where should we avoid using such words?
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You cannot take such guidelines absolutely. Good writers can switch registers for effect, but learners seldom can. The latter should try to use informal words in informal speaking and writing.
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