I have either SS bikes or converted all my geared bikes to SS........not sure why.........just love em.......
Maybe the same reason I am a telemark skier.........way less efficient skiing but I love it..........
SS as my good bike buddy says, " 75% of the time you are in the wrong gear." :-) If you are worried about hills or pulling something or having it loaded with panniers, etc. just gear it down so it is easier to pedal.......
I have strong thighs and have always been good up hill so I don't mind grinding it up or standing and pedaling.
I carry a heavy laptop in one pannier and tons of food, clothes ( I work 14 hour days so I need three meals-cheaper bringing than buying cafeteria food. Also I use reusable containers........all part of the environmental reason for biking,etc).......That is my "training." Anyway, many of us use SS bikes for all sorts of reasons.........
It can be done..........easily.........just not as efficiently (speaking for myself).......