I have gone to the MK one eveing, just before 4pm, one time, not knowing about the after hours paid event.
The crowds were lined up enmass at the entrance to be let in early at 4pm! Their event didn't start til 7pm!!!
That is what bothers me. Us regular park goers get cut off at 4 pm. I know, I know, we can stay there until 7pm In Theory!
However, the reality is that when the floodgates open for early entry, it gets unbearably crowded. All the lines were extra long. It was so crowded that we had to literally fight our way out of the MK. No kidding, we had shoulder to shoulder trying to get out of the crowds. I would never, ever do that again.
WHY do they let event goers in this early? It interferes with the regular paying day guests enjoyment, as has been mentioned time and time again on these boards. I guess the "why" boils down to what matters most lately: Money!
If that's the only time you can go to MK, then you have to realize that the crowds are awful and you will have long waits on attractions.