Can't speak to having them on mountain bikes, but as a middle-aged, out-of-shape woman, I would love some pedal assistance on my bike for hills or even just getting through red lights faster. Nothing beats time in the saddle, as they say, but it's very humbling and discouraging when one is barely able to ride a few miles and one spends their time walking a bike up what most would consider not steep inclines. It doesn't have to be all or nothing, a little boost in those rough patches would allow an older or just getting back to it cyclist to enjoy riding. And don't forget those who have/had injuries.