For one, you can just log in and update your blog. No need to be at that computer that has your SSG software installed.

EDIT: having tried hubpress just now, I take that back. I can't figure out how to post anything.

If understood this right (I didn't take a close look at the 3MB Javascript), this is SSG running in the browser, from the static site. So you don't have to run anything (except a browser) on your local machine (not even git).

Whether or not you use an SSG, I find the biggest hurdle is getting/making a good theme for your blog. It looks like this comes theme-included, which is actually a pretty big deal if you want to get up and running fast.

Yeah, Hubpress, Octopress, Jekyll Bootstrap, and at least one other I can't quite recall, are all the idea of having a preconfigured blog theme ready to go instead of having to build one from scratch (or stick with the very plain Jekyll default). They'll sometimes do things like pre-configure useful Jekyll plugins and provide additional scaffolding tools (this used to be a particularly big deal of Octopress, IIRC, for the versions of Jekyll before it added its own scaffolding system).

It's a nice to have to get started quickly when you don't want to custom build a theme or you want someone else to figure out a lot of the basic configuration.

Yes, HubPress is theme-included and the theme engine is the Ghost.io one.

As Ghost is MIT, my idea was to use the same engine, so it's easy to build a new theme or add an existing ghost theme in HubPress

Well, even if you do not understand Git, static website generators are starting to become a viable solution.. if you want to offer a web-based editing experience to your clients, you can use products like http://www.datocms.com/ or https://forestry.io/

It could be a good way to teach basic github usage.