Hopefully these facilities can consistently confront disagreeable and potentially unstable users and ensure they are discarding all needles that were used before leaving. Seattle's track record of enforcement in other areas makes me pessimistic, unfortunately.
One idea I've heard of is requiring more needles to be handed in before fresh needles are handed out as a means of incentivizing neighborhood cleanup.
I'd imagine if there was free heroin and needles inside, not many people would continue buying H and discarding syringes on the street.