I don't ride fixed and see no overwhelming reason to do so. I do ride a single speed, actually a two speed derailleurless manual shifting bicycle. As far as practicality is concerned, riding one gear ratio largely depends on the weather, your route, condition and what you want to put up with, as far as effort. My route is fairly flat and only actually 8.5 miles one way so a single speed works for me as does a gear bike. The difference is slightly and I mean slightly less maintenance. Most of the maintenance issues are in our heads. Drive trains do wear faster on multi geared bikes as far as chains but the wear on cogs is spread out somewhat. What I like about a SS is the lack of concern for the need to shift and the visual simplicity. I simply don't need multiple gears on my commute. If I have longer to go with more climbing I use a geared bike. If you can ride your route without shifting on a geared bike then find whatever gear is the best compromise and make up a SS/Fixed with that ratio. If going fixed you can usually go up to a somewhat higher ratio, I have heard.
In my experience on relatively flat to rolling terrain, a gear inch of between 60-70 inches works for me. If you are young, trim and strong you may be able to spin a taller gear. Erring on the low side will however save your knees on head wind days and force you to spin faster, to go faster or just relax and lower the intensity. The nice thing about a freewheeling single speed is the ability to coast on downgrades. This saves your energy for climbing. One thing I have noticed is that on a geared bike, I often shift up to a higher gear and end up pushing myself too hard on flats. It seems I always want to go faster and the SS stops me from blowing out my knees. I actually end up not hurting myself as much on the SS. If I approach a hill I cannot climb, I walk.......no big deal. On a geared bike I gear down and maintain my spin sometimes all the way to a 20 inch gear on long steep climbs. My SS is my everyday crappy weather commuter and while nice, I don't worry about it as much so I use it as a tool to get to work. I could just as easily use a geared bike but I have more than one so this is what I do. There is no right answer really.