My personal feelings are that so many parts of the company will have things going on all day that there won't be a lot of down time. There may not be scheduled events at all times, but I think there will always be something to do. Of course, that depends on what aspects of the company you are or aren't interested in. For example, someone who is primarily a fan of the parks or animated features and doesn't care for much else might find themselves with a lot of extra time on their hands. Personally, the only aspect of the company that doesn't appeal to me much is ESPN, but I still plan on spending some time at their booth or room or whatever they have.
I think the main reason the evening pass is offered is because most of the big scheduled events seem to be taking place during the day. I don't know how appealing the 50 and Fabulous Film Festival is to most attendees, even though it's something that I am greatly looking forward to. But for everyone else, Disneyland is right across the street and if you feel inclined to go there, it's nice to have the option with a discounted ticket. Also, Disneyland has always offered discounted after 4PM tickets to whatever conventions are going on in the area, so this isn't really anything exclusive to D23 Expo.