One thing that we have to acknowledge about Marx, even free market guys like me, is that he didn't believe that the things that we need to sustain life just get dropped in our laps without anybody working for it. There's a reason why the symbol of communism is a depiction of instruments used for hard, manual labor.
We should also acknowledge that even free market guys like me don't necessarily disagree with that phrase. Families are organized like this. Babies are less than useless to society. They're investments that you hope will pay off eventually; but, until they grow up a bit, they're just pits that consume resources without giving anything back. The parents provide in accordance with their abilities and the baby receives what it needs.
Really, so long as people live this principle as individuals without the use of violence of any kind, there's not much to gripe with. In fact, Marx did believe that communism would eventually be stateless -- he just believed that a lot of people should be murdered along the way to create his perfect people.
The thing is, it's hard to not realize that most modern, illiberal Leftists are spending infinitely more time talking about the "need" part than they spend talking about the "ability" part. Even Marx put the "ability" part first because, despite being a notorious moocher himself, he understood that what we get from people exercising their ability has to come before the part where each has his need satisfied.
If you really believe in that phrase in actionable terms, and you're an adult of sound mind and body, and you actually think that socialism has something to it as a principle, and you think that Marx might be a better resource for truth than Adam Smith, in order to live your truth, your first question should be about your ability rather than your needs.
Again, Marx didn't live a life that he deemed ideal for others. That seems common among people on the illiberal Left. They complain about people hoarding money until they have money to hoard; or, they find their own, personal, Engels to mooch off of. If you wanna be better than Marx at living the aforementioned principle, and being truly benevolent and making your contribution to the collective well-being, you should be asking yourself what your ability to contribute is first, 100% of the time, before you ask about your needs.