We've visited during the Halloween Party in the past (with parkhoppers), visiting on party days AND non-party days, and found that the MK was much less crowded on party days. I'd highly recommend taking advantage of it!
(Our gauge for saying it was much less crowded was the low waits for attractions -- in my estimation, low crowds sometimes appear higher than they really are if you're only looking at the walkways, because there are fewer people waiting invisibly in attraction queues and more people "out and about" on the pavement. So even though half of the attractions were walk-ons on party days, the park still "looked" just as crowded as it had on the much-longer-wait non-party days.) We'd go for half the day and then hop elsewhere after our midday break -- it was perfect.
