True, but that's not the audience anyways. The idea is that if you (as a web developer) want to make a portfolio site for a customer you could use lektor instead of wordpress or something like this. I generally always prefer static website generators for my own work but I cannot give it to a non technical person to fill with content. That's what motivated it.
But nobody needs to switch for anything. It's just another tool in the shed :)
[I run a Tumblr software blog at http://sandpaper.io.]
A related idea on my mind lately is using Tumblr just as a backend CMS. Then pull the content via their API to display to end users on a separate site. For example, with a bot that automates publishing to a gh-pages branch.
The motivation is that their editing tools and app are so nice, but their presentation makes a site feel less professional. (Also because of the limited abilities to tweak CSS, use JS, and optimize performance can be a bit frustrating.)