RSS.

If you're writing a blog, then you probably want a way for people to subscribe to it instead of making them visit your site manually. Unless you are confident that you not only CAN write a feed by hand, but will never forget to hand-update it after updating the post itself, you're going to want to at least have the RSS feed be auto-generated.

And once you've added the infrastructure to auto-generate the RSS index, there's very little point in not auto-generating the HTML index, too. If only because, that way, you won't forget to run it.