I'd love to see some of those studies. And what is "voluntary?" If you're a serious drug user and violent--- are you "voluntarily" homeless? 
I worked for years with that population--- and most want help, sure, on THEIR terms, i.e. let 'em stay stoned and unemployed.
"Lepers?" 
Well, would you like one of these folks--- not the false image of a hard-working person now homeless, but rather any one of what I'd call riffraff that hang around 8th and Olive--- in your living room? How about crashing on your couch or in a spare bedroom?
It's time to wake up and stop letting "advocates" define the problem. 
The vagrants one SEES hanging around downtown or holding signs are not "good folks w/temporary bad luck." They're people with long-term problems that would only get care if court-mandated. 
Again, all this pity is wasted. They've made decisions, they've alienated friend and family alike. 
Should they be allowed to gather and displace ordinary citizens?