If you go to a random park in Seattle, you'll see there are two types of parks - the ones where homeless people don't do much (yet?), and the ones where nobody else can do anything whatsoever. So I think that is a plus... saying "your park sucks because people cannot camp and do drugs in it" also sounds like lunacy to most people, so it's extra nice because you cannot make it sound like it's a bad thing, like some other exclusion policies that can be twisted and made into a strawman.
I am not sure, I think none at all. The parks I am aware of that are still usable are large and/or somewhat away from points of interest like services or density (e.g. Ravenna park that is large and surrounded mostly by burbs, where you only get yelled at by crazy people in one spot).
