I've been lurking on this site. I have enjoyed the different threads and post, and thought I'd jump into this discussion.

Posters have identified some really good reasons for going singlespeed or fixed, Toby. So give it a whirl!  Simplicity is key, but riding singlespeed and fixed, paradoxically, teaches you versatility and adaptability. that will serve you well when you ride a geared bike, similar to how mtb skills can help make you a more confident road cyclist. We have bikes for every kind of terrain and "event" and we frequently fuss about which bike for a given ride, gearing, etc... (That can actually be a fun problem!)  What happens when you take a minimalist bike (1 gear) and you try to use it on different types of terrain and for different kinds of rides? I've been riding fixed for 20 years, at first as a winter training thing (even though I have never raced), and for many years now as my "go-to" ride for almost all kinds of riding. You'd be surprised where you can venture off-road with a fixed. And when you can't turn that crank over because of a hill, hop off, shoulder the bike, and jog up the hill, like European cyclocross in the old, old days. Riding fixed can test your limits, physically. There is a minimalist aesthetic (which either you like or not). And, above all, it teaches you to make do with what you have. And that is where the fun begins because you don't know how far "you can do with what you have" until you're actually at that point.

Sorry for being a little abstract, but then again we fixed gear afficionados tend to get gooey and all Zen-like.

-- dario