Someone has to organise the unemployed. And they have to be insured. And what would generally happen is that they would replace a large number of workers who are currently doing similar jobs for the minimum wage.
Plus you've got the other problem: what do you do if they refuse / don't turn up? Cut their benefit, you will answer. But what then. What happens when they are starving and homeless because you've stopped bunging them their weekly dole? Poor health, crime and vagrancy. And these things cost money. So you save some money in the short term, and have to spend it further down the line.
I'm not against the idea that everyone should do something to help but when you have removed most of the jobs that unskilled, untalented workers used to do and replaced them with middle-management jobs, because they are more lucrative then the price you have to pay for that is subsidising a section of society that don't and never will fit in.
£70 a week, or whatever it is, is basically hush money - a fix to keep them out of the way so the rest of the society can get on and 'prosper'. It's sick, when you think about it.