hiya
There's a good explanation at
http://pedalpowerct.com/page.cfm?pageID=199

Basically I ride one for the reliability as I live in a pretty damp area (NW England) with only a 7 mile round trip to work: no mechs to clog up or indexing to drift out. You also stay pretty warm on a fixie with all that spinning, oh and it grips really well on damp/icy pavement

I could care less that they're trendy. Over here, most roadies would ride fixed all winter. Fixed Gear Gallery have a dream machine: Freddie Grubb 1960s 3-speed fixed:
http://www.fixedgeargallery.com/2004/g/hughes.htm