I put up my unpolished work straightaway, usually, on Nomesque Fiction. Some people enjoy reading a serialised form and don't care about the roughness of it all. I guess it feels more immediate? My current work is an exception, though. I got half the storyline up, then went back and added almost the same number of chapters throughout the existing text.
So yes, unplanned serials can sometimes founder horribly! I haven't used Wattpad, but I'd say anything with an RSS feed that people can subscribe to would work - that way they know when a new instalment is out.