Riding in a group with others on fixies I can see several advantages for urban riding.
1) All of the fixies I've encountered have at least 1 brake so they are street legal.
2) Being able to easily track stand is a real advantage in traffic, and fixies are much easier to do that on.
3) They accelerate quicker, especially if they were actually track frames before.

The going price for a reconditioned fixie in the Boston area starts at $300. There is a sizable market for small fixies for women and boys. While I don't see many with racks, the riders all have large Chrome, Bailey or Timbuk2
messengers bags. Most of the ones I see use Planet Bike Superflashes on the bags and a PB 1W LED headlight on the bars. About half use cages on the pedals. Simple, light fixies are much easier to carry upstairs to a triple decker apartment.