It seems this question turns into a debate every year, I was reminded about this when I read Disney's own park blog this morning:

Your Party Ticket Allows Entry as Early as 4 p.m.
Get the biggest bang for your buck by using your party ticket to enter Magic Kingdom Park as early as 4 p.m. The holiday festivities don’t start until 7 p.m., so you could use this time to get through a few more attractions.

I hope this saves us a 2013 MVMCP entry time debate.