Wasn't HTML5 supposed to save us by now??
It's always amusing to watch software companies try to shoehorn a rich client interface into a web browser. Every enterprisey management tool I've used that has both options has had a Web client that's a big ball of fail without something like Flash or Java to prop it up. Most usually roll out a web-only client for a version or two, then realize they need some client-side intelligence and use the easiest option available.
I'm not saying it can't be done (Microsoft's Azure portal is actually really nice...for a web client.) But it's rare that you see a web client that's preferred over the desktop version. I've used VSphere's rich and web clients, and unfortunately I still like the Windows client better. With 5.x, unless I use the CLI for everything I really don't have that option anymore if I want to configure anything "new." One of the nice things about a local client is feedback when you're accessing something over a low-speed or high-latency connection -- you can be sure you clicked something rather than thinking "Is that Flash acting up again, or did it actually go through?"
Maybe I'm just old, but I like the responsiveness of a GUI that has at least some local intelligence. HTML5 is a good step in the right direction, but it's amazing how much the traditional GUI application is being stuffed into the confines of a web browser.