Throw six of them up in the air, pick any one off the floor and air it. No one will be the wiser. The original Star Trek was a template series. Babylon 5 is the best example I can think of, though circumstances forced creator Joe Straczynski to alter his vision a bit. The SCI FI Channel's forthcoming Dune is another.
Serials have prestige.
NYPD Blue and ER are serials, and they win Emmys. There is also the artistic satisfaction of telling stories about characters that change and grow. To a viewer, serials certainly feel more like life. You realize, of course, that pilots for television series are filmed on more generous budgets and schedules than later episodes, typically twice per hour what the pattern for a single one will be. And so on.