You're partially right JohnD as relates to advance FP+ reservations, but this is a complaint of mine that i'm trying (and all too slowly succeeding) to let go. The advantage is guests using only MNSSHP's are able to get FP+ for attractions during a period of time they haven't technically paid for, but for which day guests have. A party ticket is for MNSSHP which begins at 7 p.m. and there is no FP+ during the party.
The argument by WDW in letting party goers in beginning at 4, three hours early, is that they can't process all party goers effectively right at 7, so they need the earlier start to get folks in. However, party attendees can currently use the MNSSHP ticket to obtain FP+ during those three hours before the stated party hours begin.
Therefore, day ticket holders who have paid for that time including obtaining FP+ during those three 3 pre-party hours, now have reduced FP+ availability and increased wait times because of it. All of this at a regular ticket price. Should it be ok for party ticket holders to be in the park and enjoying attractions, even with FP+ during a time they haven't paid for when it has an effect on those who have? Perhaps, perhaps not, but let a day guest so much as try to buy a balloon or Mickey bar without a wristband one minute past 7 p.m., and they are escorted out.
I have been to two parties in past years, and one MVMCP, and while i think they were overpriced and overcrowded, enjoyed them. Don't get me wrong, I hope all who attend this year have a blast, and based on park time available to us wish it was in the budget this year, But with parties on 4 out of 5 nights during our upcoming trip, MK hours are already greatly reduced for us, and increased wait times due to MNSSHP attendees holding FP+ does not help.