RTFs do have some formatting, yes, I’ve even seen people use TXT files in ways that work fine for reading. It doesn’t work for eBooks with a specific layout or images though.

You’re right about PDFs, but they depend on fixed page sizes which can get annoying to read on different devices since the text doesn’t flow to the screen size.

Where it falls down in comparison to epub and mobi is that it is inflexible. The various bits of software capable of displaying it are generally not available on eReaders and those programs which can display it do not seem to agree on how to do so. That’s where PDFs are much better because you can be sure that a PDF will display consistently regardless of the software used to display it.

But it’s a small point. I agree: Epub is the way to go, or mobi if you want to go the Kindle route.