I haven't been on Halloween yet so I can't give you an experienced answer but I believe the thought is that they are usually more crowded because people who don't have party tickets avoid the MK on party days because of the early closure.
On a related note, I will be there for halloween this year too, going to the party on November 1st, so I am curious about the responses as well. I am still looking for something good to do on halloween night. I'm bummed they aren't doing return to sleepy hollow this year because of the construction as I really wanted to do that.