Of course, we could argue that workers should be more motivated by the promise of a good reward, but it looks like evidence from behavioral economics suggests that fear of loss is approximately twice as powerful as the hope of gain. So let's not underestimate the power of fear to generate productivity.
By the way, I am generally supportive of the UBI, just wanted to point out a potential cost. Over the long run I think the UBI's benefits would exceed its costs, but I can't deny that there are real costs to an UBI (beyond the simple direct costs).