I can see it now: it's December and my FSA still has a balance, must be time to buy 12 Apple Watches and sell them on eBay.
This is the same with an HSA. The difference is that you can invest funds from an HSA and the growth is tax free.
I could imagine a case where I'd start an FSA. An HSA did not work out well for me a few years ago when I tried it, and would be even less likely to be a win for me now.