[susan53]

I think idioms can be very "dangerous" for a language learner. If they're either outdated, as you say, or slightly wrong, they're liable to cause amusement (which may be well-intentioned but can be embarassing) or just meet with blank incomprehension. A more serious problem can be when they're used in a stylistically inappropriate context.
If students are learning English as a Lingua Franca, or so called "international" English, they really don't matter. In fact, they are more likely to impede communication if the other participants in the conversation don't recognise them than to facilitate it. But native speaker English is highly idiomatic (much more so than many other languages) and any learner who expects to have contact with native speakers needs at least a high receptive knowledge of idiomatic expressions.
With my students, who are learning English mainly for business purposes, I teach idioms receptively, but warn them off using them unless they're 100% sure of both their accuracy and appropriacy, or in a situation (eg talking to friends rather than in an important negotiation) where they don't mind taking a risk and it doesn't matter if they say something weird.