It absolutely and totally depends on the time of year. When you go in off-peak times, EMH's don't usually affect the size of the crowds during the day and the hours are great at night, making it real easy to go through your favorite rides over and over and over.
However, it's a bit of a different story in peak times. My experience has been that EMH hasn't really affected the size of the crowds during the day, but during the actual evening EMH's the crowds have been just horrible. As an example, I was in MK in June and waited for about 35 minutes for Haunted Mansion during the day. Later that same night during EMH, the wait was roughly 55-60 min for Haunted Mansion. We found this to be the case all over the park and at other parks.
My best guess is that you have some people that go to the park that day specifically because it has the EMH later that night. You also have people that stay away from a park specifically because it's an EMH day. That seems to roughly balance each other out. When the EMH draw near, you'll find a LOT of people park hopping into that particular park to do the EMH. That makes for a VERY crowded park during the EMH.
Off peak times there are just so many fewer guests that this doesn't have as much of an impact. Off peak times are usually much cooler as well, so when you have guests that are in the park for off-peak visits, not only are they able to get through MANY more attractions in a day due to smaller crowds, they also aren't nearly as burned out and tired because it isn't so hot outside. This means that there are a lot of people that feel they've done everything (I know we all know that isn't actually possible) in the park that day and don't need to stay for EMH.
This is just my opinion though. I'm sure some will agree with me and others will not...:wave: