Many will contend that having the basics covered will encourage the poor to be more productive by not limiting their efforts to mere existence (a la "might one take time to learn programming if not obligated to a 16-hour-a-day job just to pay rent & buy food?"). Many contend this is absurd, as those not inclined toward productivity are not going to become inclined without any reason to. Reality is a complex mix of these, the overall expectation being grounded in one's faith (or lack thereof) in human productivity sans motivation.
Basic income also risks devaluing money. The purpose of currency is representation of value; if someone is not creating value, their money has no value to represent. Money taken from someone and given to others (taxation for BI distribution) gives workers nothing in exchange for their efforts; money received for zero productivity may be squandered a la "easy come, easy go". Concern is akin to minimum wage: a gallon of gas will always be worth about 20 minutes of clerking at a gas station, regardless of the number placed upon the currency used to pay for the work and buy the gas. Basic income would skew low-number pricing: higher prices may continue to represent greater value accurately, but pricing of mundane goods/services would inflate as those purchasing have done nothing to acquire the currency buying them.
Basic needs have a cost to fulfill. Making it easy to fulfill those needs without social contribution reduces overall societal productivity, rewarding idleness and punishing productivity, lowering national GDP. Requiring people to work (true welfare cases aside) encourages productivity, including in work people would rather not do but for need to provide for one's own basic needs.
Reverse question is worth asking: how can basic income possibly be sustainable?