My reason for not using one is called a cassette. One can get a phenomenal spectrum of gear choices that would allow one to climb a wall if necessary. Cogs as big as 36t and chainrings as small as 22t. And if that's
still not a low enough gear to traverse a given hill one can always resort to 'switchbacks'. They involve wheeling back and forth across the road to cut down the angle of the hill. Wouldn't recommend trying the technique in traffic, though.
