For the vast majority of riders and riding, good quality and properly set up rim brakes are perfectly adequate. Even for most 'extreme' riding they will work fine. But at the absolute far end of the conditions spectrum, like slippery roots and rocks on a steep chute down a mountain, hydraulic discs are superior.
I have been riding offroad hundreds of hours per year for more than 30 years, and I have used U-brakes, cantilever brakes, V brakes, hydraulic rim brakes, cable actuated disc brakes, and hydraulic disc brakes. THe ability to fully control speed and therefore safely traverse the roughest trails is far better with hydraulic discs. In fact, there are trials and terrain I ride regularly that would have been beyond my ability to ride safely in all but absolute ideal conditions with rim brakes.
Also, the claim that disc brakes take more maintenance or are more difficult to maintain is not, in my experience, true. Discs are easier to maintain, generally speaking, because you don't have to worry about rim trueness. Cable disc maintenance is on par with rim brake maintenance. Hydraulic discs are on the whole less maintenance because they are a sealed system, and the very occaisional bleeding required isn't all that difficult either. And pad replacement on disc calipers is generally easier than on rim brakes.