The funny thing was that I planned my dates last year (12/12 - 12/15) specifically to avoid the CM party days. Even though I'd heard that CM party days were not too crowded in the parks, and even though I don't necessarily always stay really late at DL, I wanted the
option of longer nighttime hours just in case. If I had a sudden burst of energy and wanted to stay in DL until midnight, taking photos of nighttime holiday sights, I wanted to have the option to do that. It's all about having options!
So I had my dates set and my reservation booked. And wouldn't you know, the darn CM parties moved to one week later, right in the middle of my trip!

At that point it was going to be too much trouble to rearrange my trip, and it was too late in the game to move my PPH reservation to one week earlier - all the rooms were pretty much booked.
So I went and I dealt with the 6 p.m. closure in DL and it worked out fine for me - but I think that it was okay for me because I wasn't necessarily going to be in DL until late at night anyway. I just wandered around DTD and into the enchanting GCH lobby, while plenty of people were going into DCA.
However, if you're a family/person who really enjoys staying in DL (specifically DL, not DCA) until late, then a 6 p.m. closure would make a big impact on your trip, most definitely!
We still have no clue why the CM party dates were changed last year and if they will stick to the new schedule or go back to the old schedule,
but I decided I wasn't going to risk encountering those parties again, so this year I am now going to DLR in that week-after-Thanksgiving time slot (11/28 - 12/5)! I keep hearing about how the crowd level is pretty decent for the holiday season during that week, so I am going to experience it for myself! After a few years in a row of pretty heavy crowds (getting bigger each year), I want to try this post-Thanksgiving time frame that everyone pretty much agrees is delightful!