I have a bike with hydro disc brakes and love them. I do not need them, I seldom ride when raining and hardly ever cross any water. I like them because they stop on a dime and take considerably less effort to do so, which my arthritic hands appreciate. The few times I've had to do a panic stop on my previous bike and had to grab a handful of brake I had sore hands for days, with the hydro discs I didn't. I also felt I could feather the braking more and maintain a bit more control on steep descents too, although in the Ozarks the descents I do are nothing like what other posters have cited.
So while I would heartily recommend them I would say if the bike you want don't come with them don't fret any over it, there are other things that offer more bang for the buck.