For me the web interface is essential and I think it is for a lot of other people. People like to share their life with everyone. When I go running, I post a live tracking to facebook (not necessary), and then when I finished running I post automatically the data to Facebook. I want everyone to know I just spend X calories and ran for X miles. I enjoy the fact that everyone can click on the link, and view the map of where I was running.
I'm sure this is not a requirement for everyone, but a web interface is for me.