The place where a dropper comes into its own is on rolling terrain. You can drop the post for a short descent then bang it straight up on the move to attack the rise you are rolling into. Without a dropper you are either stopping and starting the whole time and destroying your flow or you're limited to being set up for climbing or for descending only.
Believe me droppers are awesome for getting the most out of riding trails.
Finally they allow you to mess about more on the flat. If you want to manual, hop or pump a set of rollers then you can without messing about.