Why do you say that? I use a Knoppix liveCD all the time. I put my information, bookmarks and other stuff on a USB key, boot and run. When I am done banking I shutdown and remove the usb key.
As a primary OS... for one, it's impossible to apply updates, and you can't quite burn an updated CD when you're using the Live CD to begin with. You're also stuck with the inability to install anything, from programs to program updates.
The LiveCD is a nice concept, but as a primary OS... nah.