Big advantages of fix gears and single speeds as city bikes: cheaper, less desirable from a theft perspective, more reliable (especially if only ridden/looked at in the dark - a life of many norther city commuters in winter, far more crash worthy and in the case of fix gears, more training effect in fewer miles and hours riding. Also - again fix gear - better on ice and snow and works well in freezing snow, a place where derailleurs may not work at all. The drive train also works at a far lower standard of maintenance. (I used to not sweat chain lubrication and frozen links riding in Boston's and Ann Arbor's salt winter roads until the third link froze up. Just slide the wheel forward to keep the chain slack right.)

I have 3 geared bikes and two fix gears. I often change my mind after I walk into the garage as to what bike I am going to ride and it is often from the geared bike to the fix gear just because its more fun.

Ben