I've worked in homeless shelters and for public health. The major mistake uninformed advocates (not all advocates) make is they think: this is a person, just like me, down on their luck. It happens because that's how we naturally sympathize with people -- we put ourselves in their shoes.
Unfortunately that only describes a small minority of homeless people. The majority have serious psychiatric and/or personality disorders and/or drug abuse and/or health problems -- there's a lot of collinearity here too so it's hard to analyze the percent variance explained by each. Thus, their needs are different. Instead of just more opportunities (and the requisite services to bridge them to those opportunities), many are in need of a heavily structured mental health-oriented program.
In the past there was instituionalization, which of course failed because we treated these individuals as subhuman, rather than human with disability. I'm not sure what the best solution is but providing free gyms and needles ain't it.