We just got back, and if I'm not mistaken, the difference between a 5 day and 6 day ticket was something like 2 or 4 dollars. We arrived on a Sunday at 4:30pm, got checked in, and were at the AK by 5pm. They had a late night, and we did enough that we only went back to AK for a couple of hours on a later day, just to hit the things that weren't open at night.
So, depending on your specific circumstances, it could be something beneficial, for very little money.