I think the parents of pre-schoolers are imaging much bigger crowds than there will be. Remember, we are talking about the end of Aug and Sept - the time with the lowest crowds because of kids going back to school and hurricane session. Also the Magical Beginnings are three days a week in addition to the morning EMH - that means it opens early more days than not. This will spread out the crowds because many families can get up early for a couple of times, but not every day, particularly if they are staying out late. I don't think the crowds will be as big as on EMH.
One other note - I don't see it listed on the Disney web site calendar - If people don't read these boards (or other Internet web site) or have received the promotional material either directly or from their
travel agent (and these people will usually be people with pre-schoolers), they probably won't even know about it until they arrive. It is on the web page, but on a page aimed at pre-schooler parents - easy to overlook that it might apply to them if they even take the time to read it. They may get a brochure when they check in, but unless the desk CM makes a big deal of it, many will just toss it aside when they see it aimed at little kids, not realizing they could take advantage of it. You have to remember - the people who read DIS are actually a minority of the visitors to WDW, a vocal minority, but a minority. Don't not go based on the fact a few people here said they were going without pre-schoolers. I don't think the crowds will be bad and the one initial report we've had indicates that the crowds were low.
So go, have fun and don't worry about who else is having fun too.