Depends on the trail, whether I consider a dropper necessary or not. That being said, there are significant upsides to a dropper post, with minimal downsides.

I've had dropper posts for several years now on my trail/all-mountain/enduro bikes. They get ridden on rough terrain that makes a lower seat position almost mandatory...with the seat up, you're liable to go over the bars on some of the trails. Up until a couple years ago, I used to race XC on a bike with a rigid seat post. Nowadays, even my current XC bike has a dropper and I use it quite often.

For the riding I do, a QR lever is not a replacement for a dropper post. I cycle the dropper dozens of times during a ride. Even during the course of a long descent, there will be a mix of pedal-y sections and steep technical sections, each making a certain saddle position optimum.