^ this.
Unfortunately, I think any argument using this as a basis for legalising online poker is gonna ring somewhat hollow:
"People are getting hurt doing something they weren't supposed to be doing, but if we just legalised it the whole problem would go away!"
Fact is, the same argument has been used before with zero success for legalising drugs (at least, it has been here - I assume someone's tried it in the US too). The counter-argument is an easy one: "If we legalise it, people are still gonna get hurt. With drugs, they're still gonna overdose, and with poker they're still gonna lose their life savings."
I think the argument really needs to be along the revenue and regulation line - people are gonna do it anyway, but if it's legalised, at least the government can tax the income and regulate it.