Does that sound crazy to anyone else? It makes me think the authors have a hammer (i.e. web development skills) and therefore think everything is a nail (i.e. a webpage).
If I have to install software anyway, I'd much rather it be a full fledged native client. One that looks, feels, installs, uninstalls, and is configured just like all the other native programs on my machine. I had such a program in the late 90s/early 2000s. It even had support for SMIME and PGP.