my brother rides one but then he has other bikes too, people ride them for enjoyment or training or commuting. I wouldnt have one myself becaue where I live is so fucking hilly some of them are smallest gear and still a difficulty to ride (luckily they are only short).
Cycling is all about enjoyment, if you enjoy riding them then more power to you. If I still lived in my hometown I would have one because it is much flatter and would be great blassting along on one, although it would have to be a second bike because would always want a geared bike as my main bike.