Parenting is hard enough when you have enough money and no addictions. I can't imagine a scenario where you can be a drug addict or alcoholic and be an adequate parent.
And yes, there are plenty of inadequate abusive parents who do not have addictions. Doesn't mean that society should ignore what has got to be a major warning sign of inadequate parenting.
And to say that addicts should have kids or be allowed to keep their kids because it gives them hope is getting priorities wrong IMO. The only priority should be the well being of the children.
Personally I would use some sort of reversible contraceptive for addicts - it has got to be better than kids being born heroin addicted or with FAS, and more humane than taking kids away after they've been born.