It really is never a good idea to buy one in advance. As far as I know, they have never sold out, and you can buy one on the day you are there if necessary.
I can't tell you how many times I have seen people with a Flash pass, and the park was not crowded. Add to that, you are only going to be there for that day, and what if it is pouring rain? You bought a pass, and will not be able to use it.
Thanks, that's a good point. I was afraid it would be super crowded as it's the last day of Christmas break for a lot of schools, but I guess the park must not think so since they start the early closing that day.