Part of the problem is we would still have to enforce people going to the designated areas. Even with the designated areas, there will still be discarded needles and people doing it elsewhere. Its similar to how you'll often see litter next to a public garbage can.

Not saying it isnt worth it, but just the sites alone aren't enough to end the needle litter and people using in other places. We would need to encourage the sites use and discourage use/disposal elsewhere.