Knowing what a sentence in the target language ought to feel like is very useful, though. You're learning what order the words go in, how to match genders, how to express different tenses, where to put prepositions, and so forth; the point is not to memorize the specific sentences but to absorb the structure of the grammar.
When learning languages the things you should learn first are food, health and shelter.
Just by knowing a few key phrases in the main categories and the probable answers to those in a language will help you get by a lot better than "I like to ride my bicycle in the rain on the weekends" :D
Thanks - that's not a perspective which had ever occurred to me. I just assumed that learning a new language would require a long period of steady practice, and that's what Duolingo seems to be built for; but if people are often approaching it looking for something more like a glorified tourist phrasebook, I can see why they might complain about the abstract nature of the practice sentences.