MrsJackSparrow4life and Sonnyjane are right, at least based on my experience. I used a twilight ticket three years ago, when I was a convention and could only spend the evening in the park.
They are NOT a special event ticket; they are a discount/special promotion offered to conventions in town. The park doesn't empty out...it's just a discounted ticket that's good from 4 p.m. until closing.
They were a one-park ticket then, and in order to parkhop, I would have needed to buy a full-day parkhopper ticket. I decided it was worth the price savings (which was substantial) to stay in one park.
Most conventions in town offer discounted parkhopper tickets too, if that's why you'll be in town. If you want to parkhop, or if you'll be going for more than one day, the discounted parkhoppers are totally a better deal.
Hope this helps!