Lett's start with the obvious: it doesn't matter that some people will do nothing. It doesn't matter that some people will not be working. Not only are the benefits to society great (the yearly costs for society of homeless people, or similar scenarios are much greaterthan a simple $12k.), you're looking to cut off everyone from UBI because what, 5% would not be working? So, kind of like unemployment today then.
We've come far enough as first world societies that we can ensure that everyone gets a liveable amount of money every month. And don't forget that UBI cuts off every other kind of aid. Go live a life with $1000 a month, and you'll realise that you have to be in a very particular mindset to stay at home and do nothing.
In addition, it's also a great thing for the economy, as money is actually being used to pay things, and circulating taxed away from the cash hoarding dragons that are billionaires. That makes Keynes happy.