Although this year it seems every park is busier than past years, this is generally considered a good strategy, to visit MK on a party day. The reason is that party-goers cannot enter the park using their party tickets until 4:00, and many other guests who are NOT attending the party -- especially those without park hopper tickets -- will choose another park that day because of the early closing at MK.
We are arriving next week, with park days December 8-11. The ONLY non-party day is December 9. Our plan is to visit the MK on December 8, from open until 4:00, then go to DHS to see the Osborne Lights and Fantasmic! We will visit the MK on the evening of the 9th, to see Wishes and MSEP. We do expect heavy crowds that night, both because it is the only non-party night during the stretch, and because DHS closes at 6 that night for the Pop Warner private party.