My main concern with this kind of service is that most of the times I fill out a form it has personal information which I don't want to risk leaking somewhere which is why I prefer local apps that don't transmit over the internet.
It's also worth considering what you do after you've filled the form locally. Do you then email it somewhere? What does the receiving organisation do with it? Unfortunately there are potential security pitfalls in many parts of the process.
I never understood the market for web apps like this until I realised Windows must not have this out of the box (presumably?)