Sometimes they are better. If you want to perfect your pedal stroke, spend a few weeks/couple hundred miles riding a fixed gear. I like my single speeds and I like my geared bikes. In general, I think a single speed gets you up a hill a bit faster, unless the hill beats you and you end up walking.
There are times where I just don't feel like riding a bike with any level of complexity, and SS also happens to be lighter. In urban environments, or winter riding, they work well because there is no RD to get bent in a crash.
And then there are people who, for whatever reason, cannot understand shifting to save their lives.