(The same applies in other domains, too - it's fine to rely on gcc's many -W switches, but be careful with its custom C dialect; etc.)
- It's easier for you, the savvy developer who groks the toolchain, because it allows structure and reduces repetition.
- It may very well be easier for the next person who wants to add a page or change some copy, because editing Markdown and running a couple (admittedly esoteric) commands is likely still easier than editing HTML directly.
- If the next person doesn't understand or like the toolchain, they can ignore it entirely and edit the HTML directly, which is no worse than if you didn't use a generator at all.
So yeah, you don't have to use tools, and yeah, CMS sites and compile-to-CSS is probably overused in the small-site market, but I think static-site-from-Markdown with vanilla CSS is a pretty sweet spot for this!
* No web UI. This may be a blocker for business folks who want to update a blog. This is a blocker for folks who are using devices like iPad's to update blogs (which is a huge reason why I moved away from things like Octopress and moved toward things like Ghost or Wordpress.) This may be a blocker for folks who don't always have the tools on whatever machine they're using at the time in order to generate the site.
* No imposed structure. It's easier to impose a web UI form that locks folks into certain workflows than it is to just give them a git repository with all sorts of stuff strung about, with no apparent structure other than the FS structure.
I've written blogging software that did just that. Every time the author edited/added something in the web UI (which saved to a db), it just re-'compiled' the particular pages.
In terms of those downsides, I agree with you for larger sites, but not for the kind of really small sites the OP was talking about.
I suppose you could use the static generator for the initial build and then throw it all away yourself, but that's basically the same as not using it at all.