If you let people in early, you don't know when they leave, so they could stay the whole time and add congestion to the crowds throughout the day; either that or Disney wants the ability to switch from a partial blackout date to a full blackout based on the actual attendance. For example, instead of guessing ahead of time which days the park will be full and abusing blackouts, Disney can make an announcement at, say, 1:00 PM whether or not the blackout will lift at 4 or extend throughout the day.
If the whole point is to use better information to make decisions, it sounds reasonable to me.
Edit: reasonable if it's used to improve the employee benefits, which appears to be the purpose.
Or more likely it will be used to degrade the benefit and the blackout will be applied the entire year becoming an after 4PM benefit for 4 months of the year while a total blackout will be applied the rest of the year. Gotta juke those PCGS and PRGS numbers and can't have underpaid CM's diluting them for the 'Street