There are three things to consider with any gearing setup, the lowest available gear, the highest available gear and gaps between adjacent gears. You select among the options to arrive at the best comprise for the ride based on the course and rider. For the lowest gear, select one that allows reasonable cadence up any longer pitches. It's usually OK if you have to slow cadence and push hard for a minute or so up the steepest pitches on a given course. The high gear is similar that you want to be able to maintain a useful cadence on long high-speed runs. It's OK if you spin out on descents that don't last long anyway, where maintaining a good aero tuck will be faster than trying to pedal, or the recovery afforded by not pedaling with force will net a better average speed.
The reason you don't want to have a huge total gearing range is that it starts to open up gaps between adjacent gears. These become problematic on longer constant stretches where you find adjacent gears are either to high or to low for what would be optimum for you on that stretch.