Very useful! I ran into the issue Early Decision might solve when I was interviewing for the YC Fellowship program. Our interviews were 2-3 days before my fall semester was to start. I had, at best, a 24 hour turn-around time between getting accepted and going on school-leave. Very little time to weigh the decision to join or not, and logistically stressful getting all the documents in order to make sure I didn't incur tuition costs for a partial semester.