I ride in NYC about five miles every work day, from Penn Station to my office and back again. I used to ride a Strida (one speed) but now I ride a Downtube Mini and I use all of its eight speeds (well, I probably don't always get up to 8th). I'm sure you can get away with fewer, depending how you ride, but the 8 speed hub is such a good suit to the way I ride that I've come to regard it as a necessity.

The important thing, if you have only a few gears, is that they're the right ones. I test rode a 2-speed Brompton once, and found it geared too high, which makes it feel heavy and sluggish. But again, it really depends on the way you ride.