Geared bikes are safer than single speed bikes (and much safer than fixies) in traffic, because they can accelerate much faster from a slow speed or standing start (unless you use a single gear of 43" or less, in which case you'll spend most of your ride coasting) . This can be crucial for when making difficult turns, getting away from traffic lights before the cars, or responding to a suddenly perceived danger.

They're also much easier on your knees.