Yes -- you are sort of kicked out of the park at 6. Have to show a party wrist band to get to certain areas, etc. For the night time entertainment this doesn't work. I believe that there will be a parade you can see before 6. And things get more crowded at 4 p.m. big time when party people come in. Check the park schedule. You will not though without a party ticket on a party day get to see the fireworks from within the park.
Now and this is just me (a rare person who doesn't care about the night time entertainment), since almost everyone else who goes to the MK does care about that, regular tickets to the MK on party days mean the park is way less crowded than normal which I love. It's a favorite day for me to go during the day, a very rare opportunity for lower crowds. Usually though I am with others who really like those nighttime shows, so I only got to go during the day of a party day once. It was wonderful. Crowd calendars show that this is consistently the case with low crowds on party days and very high crowds on non-party days as people want the longer hours and the fireworks. The parties really skew the crowds, and I personally feel like Disney is double dipping getting two sets of admissions on one day.
Many love the parties, but for me - meh. It was for me a one and done for the MK Christmas Party -- way too crowded.