I think the problem with that argument is that - sure - layabouts etc. will always exist to some extent. But you shouldn't under-estimate the extent to which people naturally change their behaviour according to the social expectations placed on them. I think if you create a system of UBI in which people are always given enough to live on with absolutely no expectations that they will do anything in return, you can guarantee that the number of 'layabouts' will soar to massively greater numbers than currently exist - because of people unconsciously reacting to the lack of expectations placed on them. And that could be very harmful to our society. To my mind, a healthy society really does require an implied 2-way contract - in which society/the Government looks after people: But, in return, people are expected to be willing to contribute to society. The danger is that UBI would destroy that link.
For that reason, I'd prefer any UBI to take a form more like 'guaranteed work': In other words, anyone who wants it is given - say - 8 hours guaranteed work a week, provided by the Government and paid at the minimum wage. I'd even go as far as to say that maybe that should be the gateway to all other benefits: You can apply for other benefits IF you're doing your guaranteed work and you can show you still don't have enough money to live on. Obviously there would be a lot of details to work though to make that work properly (for example, you'd need to provide childcare for parents. I'd also make sure the scheme is available to all homeless people - with accommodation thrown in). You'd also need a humane, sympathetic, system to assess people who apply to find suitable things they can do, and may even to allow for people to spend their guaranteed hours in training if that's appropriate to them. (Lack of a humane system to assess people seems to be where most Tory welfare reforms dramatically fail

) I think that kind of system would give people the guarantee of support when they need it, but without breaking the social contract of, 'society looks after you iF you are willing to play your role in the community'