The virtue of such a policy is that it increases the salience of the cost of policing immigration, and so it could increase opposition to closed borders.
And this cost is already significant. The UK Border Agency cost £2.3bn to run in 2011-12. And those who think illegal immigration is a problem presumably think this sum was insufficient.
But £2.3bn is equal to almost half of spending on Jobseekers Allowance in 2011-12.
Let's put this another way. There are now 463,000 people who have been unemployed for over two years. Let's assume half of these are "scroungers" who could - if they wanted - find full-time minimum wage work. (I think this is an absurd over-estimate of their chances of getting work, but humour me.) How much would this save the tax-payer?
We'd save JSA of £71.70 per person per week. That's £865m. These people would also pay just over £1200 a year in tax and NI, which saves the Exchequer another £300m. There'd also be a saving in housing benefit. Let's call this £400 per person per month (the rent on cheap one-bed flats in Leicester) which is £1.1bn. This gives us just under £2.3bn.
In other words, the cost to the tax-payer of policing our borders is roughly the same as a generous estimate of the cost of "scrounging."
You might object that policing the borders saves the taxpayer money to the extent that immigrants would be a drain on the welfare state. You'd be wrong. Most estimates show that immigrants are a small net benefit to the Exchequer.
Personally, I think these sums are small. They are less than 0.2% of GDP, and there's a lot of ruin in a nation. The biggest costs of border controls are the loss of liberty, the reduced welfare of would-be immigrants and the economic benefits we forego.
However, my point is that anyone who is annoyed by the cost to the tax-payer of "scroungers" should be irked to a roughly equal extent by the cost of border control.
But I get the impression that very few supporters of UKIP or the Tories are. Which makes me suspect that what's at issue here is not a narrowly rational calculation of fiscal costs and benefits.