Do you learn languages for the sake of learning languages, or for another purpose?
If you learn because you enjoy simply enjoy learning another language, or love the beauty of that language, then you are 'learning for learning's sake'.
If you are learning because you have to speak another language for work or for travel, then you are learning for another purpose.
Most people need a purpose for learning a language, and do not learn just for the sake of learning. But once they start and they enjoy it, then they might say: I learn English for work, and also for learning's sake.'
In this case 'We do not want prohibition for prohibition's sake (because eg. drinking alcohol is morally bad)' we need to consider the other important factors so that we can make a good decision.