My half-arsed official reasons:

- I can ditch both derailleurs/cables/shifters, and I don't need to adjust anything
- It won't derail when the cogs get jammed up with snow this winter
- I've been contemplating going single-speed for a while now, but I do have 2 bits of hill going to and from work that I didn't want to tackle with only one gear.

My real, hidden reasons:

- It's different
- I really wanted to try pedalling a bike backward
- I expect the bike to be a great conversation starter
- I had nothing to do this week-end