There will always be a portion of people who will not want to participate with the majority or who will not want to work (if they are only willing to do meaningful/interesting work, that's a different matter). In our modern system and with basic income these people would be able to still participate in the system and in consumption. This would keep the economy going by providing bigger markets for companies to provide products and services to, so they can grow.
The other option would be to cut people off, take a hit to the economy (which it couldn't take) and risk these outsiders gathering and collaborating. Personally I wouldn't consider this a risk and I'd consider it people their right to compete with this system in however way they see fit.
Just consider that they may prefer to compete in a violent way and just consider that something like social security is pretty much that, security spending. It keeps people from acting out and rebelling because they can at least get by. I think they should get more and I'm strongly for basic income (especially because it will really be needed in the future). But to all those who think that for some bizarre reason people without jobs are lazy and don't deserve a decent life or should be forced to work, keep in mind that things social security, benefits, food stamps and whatever also prevents uprisings because it takes away many people their motivation to rise up.
Even to crazy people who can't see other perspectives, or who oppose socials support (or basic income for the future) this should be some argument.
Regardless, slavery is unacceptable. To anyone who thinks it is, keep in mind it would lead to uprisings which would cost you in terms of higher security/law enforcement spending, and of course less stability (economically very costly).
