Casey Liss on third-party complications:
Not having used a Watch for more than a few minutes, I don’t really know how glances and apps will fit into my life. Nevertheless, in many cases, I can think of scenarios where just a few characters of information may be enough to convey all I need to know. Perhaps in much the same way that your next calendar entry can be shown in well under 50 characters.
Imagine a way in which just a few characters of information can provide everything you need to know about something. It wouldn’t require any interaction with the watch; simply raising your wrist and looking down would get you the information you need.
Casey goes on to list several potential uses for complications and points out that Jason Snell has also been thinking along the same lines.
Third-party complications are a no brainer — as long as Apple believes it can continue offering reasonable battery life with them enabled. But, I think it’s only a matter of time before they finish up the APIs and hand it off to developers.