^^ I think the four parks worth of people condense into one is a bit of an overstatement. Most people with children won't make use of the EMH, so the crowd during EMH is definitely lighter than the one right before EMH starts.
Uh, how did you get "four parks worth of people condense" out of "which means that you have four parks worth of people who tend to gravitate towards whatever one is open latest for them." "Tend to gravitate" is the key term there, and it's fact.
I have to disagree completely with you when you say the crowd for EMH is lighter than before EMH begins. I almost never see that to be the case. While not EVERYONE does this, the prevaling wisdom seems to be that more people gravitate to the EMH PM park "because they paid for it", i.e. the EMH feature by staying on-site, which artificially inflates the amount of people who would go to that park. The night before, and the night after (and ESPECIALLY the morning after) an EMH is almost always much slower because so many people plan that as their "MK NIGHT" because they blindly follow what the resort tells them their "benefit" hours are.
It's the same phenomenon as how the BUSIEST any park is going to get on a given week is the mid-morning of an AM EMH day. Tons and tons of resort guests designate whatever day is that parks AM EMH, again, to get their "benefit", but like many on a Disney vacation they oversleep, don't make the actual hour you get the benefit, and end up going to the designated park anyway that day because it was in their plan. That's why smart people get up early, get to EMH and are riding rides at 8:01, and see half the park before lunch when most people are just seeing their first attractions.
It is not overstating it in the least to say that EMH inflates crowds because resort guests will always tend to gravitate toward whatever park is latest. Not *everyone* does it, but many do, enough to make an impact. And I'm not sure how many EMH's you have been to, but it has not been my experience that parents with kids avoid them...they may be dragging the kids along half-asleep, but they are still there, "getting what they paid for".
AEfx