I have difficulty using web-based apps because they are just too slow for me. I am a bit of a speed freak, tweaking and overclocking, etc. -- but I do value my time and I've never used a web-based app whose speed pleased me.
If I have a choice, I'll stick with desktop applications forever.
Sure, they're the right solution sometimes. But that doesn't mean I want to use them if I can help it.