I am convinced of the importance of learning useful set phrases at an early stage of language acquisition. Moreover, I believe that learning humorous, rare, idiosyncratic, and slightly odd words and phrases is extremely useful, too. Although these cannot be used in any circumstances, they would often open many doors - and hearts - of native speakers.
I am able to engage in more satisfying conversations with Greeks, when I reveal that -- apart from everyday's kalimera/kalispera -- I know to say palikari (a compliment to a Greek macho), briki (Greek coffee pot), phrontistirio (private school, the word originates from Aristophanes where it meant the 'thinking house'), and kouraviedes (my favourite biscuits).
Funny and odd phrases, being individual and easy to remember, can be also handy when you need to master complex grammar features. I learned the entire set of rules on the use of Italian subjunctive by making up sentences involving my flames of the moment. Seven years on, I have no idea where all those flames have gone, but I still remember by heart these rather obscene -- and mostly written in highly hypothetical or unreal mode -- sentences. Needless to say, I rarely make mistakes in Italian subjunctive.
I have a favourite phase, which I try to learn in any language, even in those I will never be able to speak: You have beautiful eyes. I must confess that few of those whom I have asked to teach it, be it in Armenian, Hebrew, or Hungarian, reacted adequately. Many were surprised, some shocked, but in some cases it was a true ice-breaker.
And I am always happy to learn a few phrases in any language, so when I meet someone who speaks a language new to me, I ask them to teach me some. My most recent attempt was at a Bulgarian colleague, and the response was more than adequate. 'Yes', he said happily, 'I can teach you Would you like a cup of coffee? and Would you like a massage? My collection of useful useless phrases is expanding!