I'd be glad to see any definite information you might have on the homeless and their income/spending habits. We're not talking about the sky and the grass Rightie, but people.
You say "millions" - how many "millions"??? What is their income? How much do they spend now? Where does the money come from? Do the live singly or in pairs? How many have a permanent residence? How many are illegal aliens?
I'm sure any studies you might have will answer these and other questions. I'm afraid your just claiming it won't do the job. And I'd point out that living below the poverty line still has that person paying taxes on each and every taxable thing purchased. That's still more of a contribution to tax revenues than they make at present - by a good deal undoubtedly.