Neat and orderly chainline: On a load-carrying bicycle.
The chain broke on my S.U.B. I fixed it. It broke again. By now the chain was getting so short that splicing it again made no sense. So I removed the rear derailleur, adjusted the chain length and set the bicycle up as a single speed, right there in the logging road's ditch.
Here's a strange thing. I don't miss the gears all that much. Even on a load-carrying bike used in seriously hilly territory. Sure, my whip is now sort of slower downhill and on flats, but surely this lowers the chance of hideous accidents and self-mutilation. And some of the speed I lose downhill is compensated by increased velocity uphill, as I no longer have access to the granny gear and am forced to just step on it.
There's a whole slew of benefits with a single speed. Much, much less maintainence, for example. Less small, exposed parts to worry about. No twisting and less friction on the chain, making the transmission more efficient. There is also the issue, on this kind of bicycle which has been elongated with a Free Radical from Xtracycle, with chain slack, which is now omitted.