Insite isn't keeping these people out of prison, nor is that facility the only place where an individual consumes their drugs. Society won't be getting rid of drugs anytime soon, so like it or not, we're stuck with the consequences.
In the end, it really doesn't matter whether the "solution" is prison, forced rehab or voluntary access to medical assistance. The probabilities are such that the longer an addict participates in this activity, the more likely they are to die from medical-related complications, violence or overdose.
That is a cold and clinical thing for me to say, but no matter how much you pretty-up the issue, it's nothing more than a form of prolonged suicide.