I think the most compelling reason is the list of extensions. They integrate several missing functionalities from webapp; you just pick what you need. You could

I think the most compelling reason is the list of extensions. They integrate several missing functionalities from webapp; you just pick what you need. You could also build them around webapp -- that's what we are doing with tipfy. I wrote a list with some other reasons: http://www.tipfy.org/wiki/why-tipfy/ http://www.tipfy.org/wiki/why-tipfy/