I have an Oxygen eMate MTB. 90% of the time, when I'm commuting, I'm on the largest front chainring. However, when I go onto bridleways and encounter steep bits, having the option to change down to one of the smaller two rings is very useful. As was mentioned earlier, if the battery expires out on a ride, you need the lower gearing.
Having just had my bike revitalised by fitting a new drivetrain, ensure you keep the gears well maintained. It is annoying to have gears that won't change when you want/need them to, especially on rough stuff.