There have been multiple studies comparing the effectiveness of manual
brushes as opposed to electric brushes.
Also, some electric brushes (Sonicare) produce sonic vibrations that are difficult to mimic with a hand brush! Other electric brushes like Oral-B and Rotadent have small heads that help you access hard-to-reach areas of your mouth. This aspect is more important when you are talking about someone with orthodontic braces or a history of gum disease.