Yes, the only downside I can think of is if you have an HSA (which anyone that can afford max out of pocket limit should), then you generally cannot have FSAs also unless they are Limited Purpose FSAs and then it gets too complicated for my tastes. I generally do not like the concept of FSAs period, being employer owned, and having to use up funds by the end of the year and all.

Agreed. I only put money in an FSA that I know I am going to use. But for my routine cleanings/xrays/etc, this is pretty easy to calculate.