Incentives. Incentives. Incentives.
People respond to incentives, it's been proven time and time again. A negative income tax isn't the worst idea, but it has all the wrong incentives. The more money you earn, the less help you get. Which seems sensible on the surface but can lead to situations where people ask themselves "why should I even bother working a job I hate when I can get the same money without working?"
The advantage of basic income is that no matter what you do you still get that money. You're still incentivized to work because a life on basic income won't exactly be a life of luxury. In fact you might be more incentivized to work for several reasons. One because working will get you money you can spend on stuff you want, not just stuff you need to survive. This is significant from a quality of life aspect. Also, working will enable you to get ahead, instead of just being on a treadmill as so often happens today. You'll be able to have more choice where you work and how much, and you'll be able to set aside some of your time to pursue new careers.
I don't think people understand just how much potential and talent is being wasted and misused in our economy at present. Human beings aren't meant to be drones, that's not our calling. And it's an insult to the human race to imply that a significant portion of the population is somehow genetically fit for nothing other than menial labor. That's never been my observation. In the vast majority of people I've met there is almost always a depth and a potential that is not being tapped by the current economic system. And you see some of this coming out with new economic models enabled by the internet. More freelancing, more startups, more people engaging in their passion projects through crowd funding or what-have-you. More people learning new things and trying new things because the internet makes discovery of knowledge a lot easier. That's true of musicians, artists, inventors, and so many others.
I can only imagine the world we'd have if we could unlock that huge untapped potential.