Part of the argument is that this isn't disposable income, that it's in fact needed as a baseline against poverty.
The lack of a meaningful definition shared by both law and society is the root of much of the consternation over the topic.
People working at minimum wage can find themselves in such situations.
At this time, you can get ahead on MW, if you team up with other MW workers to live together and one person can get enough "family" support to get to the next level. However, our society, being very individualistic, doesn't really recognize this.
It's not a matter of opportunity, it's a matter of choice.
Time and again I've watched those with opportunity squander it, while others lacking opportunity fight to get it (and got it). I've had a 4AM bus driver take my evening C++ class, passing despite often dozing off therein, finding funding despite not having the steep tuition. I've known a chronically bedridden invalid who managed to run a profitable, um, pharmaceutical service from his gurney. I've worked multiple jobs and still learned new skills, even getting hired despite not having the core skill and learning it sink-or-swim. I've watched kids destroy windows while I was fixing their couch. I've seen eminently capable students with amazing affinity to programming fail a beginning C++ course for lack of effort, not understanding. Opportunity IS there; taking it or making it is up to the individual.
Instances of true "poor for lack of opportunity" are exceedingly rare. They have my sympathy and aid; I'm frustrated by the vast numbers who insist they are and in truth aren't.
http://en.wikipedia.org/wiki/Disposable_and_discretionary_in...
disposable = wage - taxes
Therefore disposable income pays for rent, food, gas... All of which would be needed to alleviate poverty, but I guess that depends on how we're defining poverty.