Because I believe that privacy and the cloud can co-exist in a mutually beneficial fashion. Just because the app is online doesn't mean that it has to be "social" (there is intentionally no social-sharing options for the log or journal entries). But by being online, the application itself can improve over time by analyzing aggregate data to detect anomalies (like Mint.com), give predictions, refine the natural language parsing etc.
If this was just a journal, I would agree with you and make it self-contained. But it's not. In the next 5-10 years we're going to be seeing a massive number of wearable sensors embedded in our clothes, appliances and tools. A managed, online API will be better able to be always available, consuming this tsunami of data and turning it into useful insights for you.