I enjoy single speed bikes but additional speeds are also nice especially on extended grades, headwinds or both. In the winter I run my Brompton as a single and in warmer weather I go back to a 3 speed, or if serious climbing is likely I add a 38 tooth chain ring for greasy finger shifting, giving me about 5 speeds considering overlap. The extra gears usually let me go a little faster so I will extend my ride further in nice weather.