Oh .. I am sure there are PLENTY of people that stay on-site for EMH. But I would argue, that is the minority.
I would argue that offering a 4th Fast Pass would get just as many heads in beds, if not more ... .especially to NEW or first-time guests. I mean there are people who choose to stay on-site now JUST for that extra 30 days to snag the 3 Fast Passes that are offered. (In some cases it is the only way to get Fast Passes for the "hot" attractions.
What is an easier "sell" to a new visitor? Get a 4th fast pass to skip a fourth line every day of your trip.
Or getting the option to go to a park(you maybe weren't planning on going to that day) super early or super late? A lot of families will just go "meh" .. we aren't early risers and we don't want to stay late and that' won't be a factor in their decision to stay onsite. They may use EMH, but won't be part of the reason they made that decision.
My sister-in-law and her family came to WDW last summer. I (a veteran), noticed the day we planned on going to MK was a morning EMH. I told everyone we were getting up early to make it there. (Got there (with my wife, 3-year old son and grandparents) at like 7:50 AM). We had a blast that first hour. My sister-in-laws family (two toddlers) said NO WAY .. we aren't getting up an hour early to get to the park by 8:00 . .and they rolled in at 9:30 that morning. EMH was not the reason they chose to stay on-site.