It is a mistake to assume that those living on UBI and not working in formal employment would be "never productively contributing to society". Sure, a portion of UBI recipients might be entirely idle, but others would contribute to things that are seen as a social positive even if they don’t neatly fit into profit-generating, paid-salary-position-creating activities.

For example, if I didn’t have to work myself, I could finally do some of the more ambitious contributions to OpenStreetMap that I have been dreaming of: adding all the missing house numbers for my county, for instance. Then society as a whole benefit from that libre resource even if I as a contributor was not paid a salary for it. By the same token, some of the most active Wikipedia editors are able to dedicate such enormous time and effort because, for whatever reason, they do not work. Yet all of us here benefit day in and day out from what they do.