In this case, letting millions suffer in poverty with real effects of poor healthcare (instead of investing in preventative care), restricted access to better opportunities for themselves and their children seems thoroughly worse than accepting the outlier "parasites."
I am of the belief that given the foundations of Maslow's hierarchy and a real education, many of those "parasites" with limited opportunities can be changed into people who feel they have a chance and pursue "self actualization." Poverty is a vicious cycle; it's hard to be ambitious in a "i want to change the world" way when you have no choice but to take whatever you can to support your family on minimum wage.
I agree completely that instead of throwing money at administrative peripheral problems like eliminating any parasitism, we should address the root problem.
Overall, people living in poverty do not have the same opportunities as the wealthy. Given the same opportunities there is no reason that they would not pursue the same "worthier" career aspirations. The assumption that poor people are parasites is the most colossal example of Fundamental Attribution Error[1] I can think of.
[1] https://en.wikipedia.org/wiki/Fundamental_attribution_error
This is the basic issue every argument against any social program needs to address.
The last time basic income came up I saw a poster strawmanning it by claiming that the "producers" would be financing everyone else to have daily parties. Actually nobody is suggesting that we give enough money for people to throw parties every day. The suggestion is to give people enough money to survive in a way that eliminates government waste on the program. Then we will see what sort of jobs or tasks they create for themselves. The majority will not be content to watch TV all day.
Today, means-tested aid receives a huge amount of criticism about supposed "leechers", even though abuse is basically a rounding error, and the vast majority of recipients are not at fault for their situation.
Imagine how much worse the criticism could get when there really are leechers because leeching is officially approved.
So no matter how nice a BIG is, I often wish that at least part of the massive amount of political energy spent on promoting it would instead be targeted towards poverty-reducing and power-shifting policies that have a higher chance of being implemented and remaining implemented.
Edit: man, typos galore. How embarrassing.
One political problem with welfare schemes is getting over the psychological hurdle that hard-working people are funding welfare abusers. I, for one, do not care if there are a few egregious abusers if the overall system is cost-effective.
Focusing on eliminating parasitism leads to wasting even more money on administration, e.g. drug testing for welfare recipients.