Opponents to basic income argue, among other things, that this is too much power and some workers would become exceedingly picky, without the fear of not being able to pay for rent and food. There definitely is a moral aspect to the question, with "anti-cheater" emotions/reactions.
I think that's important, and not just for those people: that they don't have to accept a minimal (and likely dead-end) job to survive also means that those who would be inclined to are not prevented from taking steps that prepare them for a better and not-dead end job, that is more useful both to them and society.