At the past ESL NY event, thy had the two semifinals played before the finals. This cause it so that both teams in the finals ended up playing six games total that day, with VP the worse for wear as they had to play against SK in one of the best sets of the year and then move on to play NaVi. NaVi was also tired as they played Liquid earlier, but they got a longer break in between.
What people should think about is that mental endurance and mental focus are limited resources. Very few people can concentrate 100% on any one thing for a prolonged period of time. You can try it out yourself. Close your eyes and count to 1000 without thinking about anything else. Now what you did before, not what you did after, not what you have counted already or what you will count. Just say the numbers and you’ll find that task much harder than you’d imagine.
In competitions, mental focus can be the difference between victory and defeat (just look at soO and Mvp’s careers to see the difference). This is why finals should be scheduled on their own days as they will allow players to be at their sharpest going in (exception being FGC games as those guy by really fast).