In my experience the reliability of single speed bikes is overstated. Several times when I rented or borrowed single speeds the chain kept dropping when I pedaled hard over rough roads. Sure that problem can be prevented through proper maintenance but in some circumstances it's sure nice to have a proper derailleur maintaining chain tension.
The chain should be tensioned enough to be impossible to fall off, so those bikes were just not set up correctly. But that is still a good point about problems people can have with bikes.
