There's a lot of great stuff that could happen if people weren't chained to wage slavery and could lean on UBI to pursue passions. Sure there will be people that sit on their ass and do nothing, but there will also be people who use their new found freedom to create great things that wouldn't have happened otherwise.
I'd also like to point out that even the most radical UBI proponents aren't pushing for a truly substantial benefit. Most of the proposals I've seen are payments that would be near poverty level if it was treated as a sole income for any length of time.