Been using one since the original Gravity Dropper came out. Wouldn’t consider a fixed post ever again. It was a real game changer for me on gnarly steep single track. They were a novelty back then, but I’m not surprised droppers are now pretty much standard issue on serious trail bikes.
Think it depends on a few factors, mainly how much steep descending you are into and how tall you are or more specifically how long your legs are. I’m quite tall (6′ 1″) and have long legs, so my optimum seat height for pedalling is much higher than I would prefer even on relatively tame descents. On steep descents it’s dangerously high. So without a dropper I would be forced to manually drop the seat quite often. My new bike came with the Specialized 3 position dropper, which works perfectly for me.
Worth mentioning that I find droppers really good even on moderate downhills when you are charging through a series of tight turns. It’s like having a 5″ lower BB without smacking the pedals.