Separate to how useful this is, this is an awesome marketing technique that others should look into. That is, distributing a desktop app that's really just a front-end for what your Web app already does.
I've noticed it in myself (and I'm sure it applies to others) that I sense more "value" in downloading a "free app" versus using a free Web service. I was familiar with Litmus's services before, but I hadn't really bothered to give it a go until this. This desktop app makes it convenient (even though you have to use the Web to setup an account!) and it "feels" better somehow. If you have a Web app, think about whether a similar tactic could work for you (I've seen a similar thing happening with free iPhone apps promoting Web apps).
Pretty ironic, considering usually an argument for free websites over free desktop apps is that it's easier and less dangerous to try out a free website because you don't have to install anything.
I think it taps in to old assumptions. "Software expensive, Web free." On 37signals Live yesterday David Heinemeier Hansson was talking about how people take recommendations more seriously when someone is a paying customer of something than just a free user. I think this sort of thinking ties in with software being perceived as more valuable and why Alkaline could, perhaps, get better buzz than the underlying service.
I've noticed it in myself (and I'm sure it applies to others) that I sense more "value" in downloading a "free app" versus using a free Web service. I was familiar with Litmus's services before, but I hadn't really bothered to give it a go until this. This desktop app makes it convenient (even though you have to use the Web to setup an account!) and it "feels" better somehow. If you have a Web app, think about whether a similar tactic could work for you (I've seen a similar thing happening with free iPhone apps promoting Web apps).