Just discovered this thread... until not very long ago, MTB was a totally foreign world to me. I'd never even heard of dropper posts, and when I did, I wondered what was the point of them.
Now I have an MTB and I was glad of the dropper post on this morning's ride in the woods, which included a few flattish bits, a short road section, and a long uphill slog followed by a "gnarly" descent. And the more I get into MTB riding, the more I totally get why downhill racers have their saddles set so low. Only problem is I lack both the upper body strength and technical bike handling skills to descend at any real speed with confidence. Being able to get the saddle out of the way does let you go a bit faster though.
I note that the new Shimano gravel groupset includes a switch for a dropper post in the left-hand lever on the 1x version, which is intriguing.