I've heard that as well, claims that cats are second only to man as the most destructive of species.
They've definitely done their share of damage to small ecosystems when introduced.
We have three of the little devils, and I'd hate to see them let loose in the neighborhood. The squirrels, rabbits, and birds that take refuge in the yard wouldn't have a chance in other parts of town.
You can go for a walk and see a cat running loose in every second yard in some areas, and the constant howling/screeching at nights is nuts. We just don't see as many because it's mostly a dog neighborhood, with several large breeds left out in fenced yards 24 hours a day.
Hope the poisoned cat gets better and returns to its rightful place on the window sill