Not really my area, but I'd think the advantage IS that you don't need to do a local client install. Have a new user? Send them the link, and they're up and running using one of your floating CALs, or whatever they're called. Upgrading to the latest version of QlikView? You don't need to upgrade any user PCs. I know in our shop, for instance, we've done a poor job of keeping everyone on the same version of QlikView. I'd think that problem would go away if we were using the AJAX client.
Mind you, I took a look at some of my applications in AJAX a while ago, and they were totally unacceptable. But if I were designing specifically for it, I probably wouldn't have that problem.