Humans evolved needing to work to survive. It's in our nature to become restless and sociopathic if we have nothing to do.
Additionally, I don't think the idea here is to encourage idleness. It's about people not needing to either beg an existing profitable business to take them in or create one ex nihilo in order to survive.
Just the idea that the alternative to jobs is idleness shows a strange conflation of ideas. We define "work" as that which we hope will earn us money — we know an activity is work because of a totally extrinsic factor. There are people in our society who are paid a king's ransom to literally play games, and there are people who do back-breaking labor without anywhere close to fair compensation, and this is all work because they make their living doing it. But if somebody (say, a painter) doesn't have customers or an employer, that's not work, and thus it looks like idleness, no matter how engaged they are in the activity.
With mincome, if the basic needs are paid, then people could do other non-economic activities, such as more dedication to arts and culture, develop studies, etc...
For example, I believe a lot of people would increase participation on open-source projects in internet